The Secret Source for Hoshana Rabba
מתני׳ מצות ערבה כיצד מקום היה למטה מירושלים ונקרא מוצא יורדין לשם ומלקטין משם מורביות של ערבה ובאין וזוקפין אותן בצדי המזבח וראשיהן כפופין על גבי המזבח תקעו והריעו ותקעו בכל יום מקיפין את המזבח פעם אחת ואומרים אנא ה' הושיעה נא אנא ה' הצליחה נא ר' יהודה אומר אני והו הושיעה נא ואותו היום מקיפין את המזבח שבע פעמים בשעת פטירתן מה הן אומרים יופי לך מזבח יופי לך מזבח ר"א אומר ליה ולך מזבח ליה ולך מזבח כמעשהו בחול כך מעשהו בשבת אלא שהיו מלקטין אותן מערב ומניחין אותן בגיגיות של זהב כדי שלא יכמושו ר' יוחנן בן ברוקה אומר חריות של דקל היו מביאין וחובטין אותן בקרקע בצדי המזבח ואותו היום נקרא חבוט חריות מיד תינוקות שומטין את לולביהן ואוכלין אתרוגיהן:
MISHNA: How is the mitzva of the willow branch fulfilled? There was a place below Jerusalem, and it was called Motza. They would descend there and gather willow branches [murbiyyot] from there. And they would then come and stand them upright at the sides of the altar, and the tops of the branches would be inclined over the top of the altar. They then sounded a tekia, a simple uninterrupted blast, sounded a terua, a broken sound and/or a series of short staccato blasts, and sounded another tekia. Each day they would circle the altar one time and say: “Lord, please save us. Lord, please grant us success” (Psalms 118:25). Rabbi Yehuda says that they would say: Ani vaho, please save us. And on that day, the seventh day of Sukkot, they would circle the altar seven times. At the time of their departure at the end of the Festival, what would they say? It is beautiful for you, altar; it is beautiful for you, altar. Rabbi Elazar said that they would say: To the Lord and to you, altar; to the Lord and to you, altar. The mishna notes: As its performance during the week, so is its performance on Shabbat; except for the fact that they would gather the branches from Shabbat eve and place them in basins of gold so that they would not dry. Rabbi Yoḥanan ben Beroka says: There was a unique custom on the seventh day. They would bring palm branches to the Temple and place them on the ground at the sides of the altar, and that seventh day of Sukkot was called: The day of the placing of palm branches. Immediately after fulfilling the mitzva of taking the four species on the seventh day of the festival of Sukkot, children remove their lulavim from the binding and eat their etrogim as an expression of extreme joy.
אתמר ר' יוחנן ור' יהושע בן לוי חד אמר ערבה יסוד נביאים וחד אמר (ערבה) מנהג נביאים תסתיים דר' יוחנן הוא דאמר יסוד נביאים דא"ר אבהו א"ר יוחנן ערבה יסוד נביאים הוא תסתיים
It was stated that there is a dispute between Rabbi Yoḥanan and Rabbi Yehoshua ben Levi. One said that the mitzva of the willow branch is an ordinance of the prophets, as Haggai, Zechariah, and Malachi instituted it in the Temple as obligatory. And one said that the mitzva of the willow branch is an ancient custom practiced by the prophets and adopted by others as well. It was not instituted as a binding ordinance. The Gemara suggests: Conclude that it was Rabbi Yoḥanan who said that it is an ordinance of the prophets, as Rabbi Abbahu said that Rabbi Yoḥanan said: The mitzva of the willow branch is an ordinance of the prophets. The Gemara concurs: Indeed, conclude that it is so.
אמר אייבו הוה קאימנא קמיה דר"א בר צדוק ואייתי ההוא גברא ערבה קמיה שקיל חביט חביט ולא בריך קסבר מנהג נביאים הוא אייבו וחזקיה בני ברתיה דרב אייתו ערבה לקמיה דרב חביט חביט ולא בריך קא סבר מנהג נביאים הוא
§ The Gemara relates that Aivu, father of the amora Rav, said: I was standing before Rabbi Elazar bar Tzadok, and a certain man brought a willow branch before him to fulfill the mitzva. He took it and waved it; he waved it and did not recite a blessing. This indicates that he holds that the mitzva of the willow branch is a custom of the prophets and is therefore performed without a blessing. Similarly, the Gemara relates that Aivu and Ḥizkiya, sons of the daughter of Rav, brought a willow branch before Rav to fulfill the mitzva. He waved it; he waved it and did not recite a blessing. This indicates that he, too, holds that it is a custom of the prophets.
יסוד נביאים הוא - תקנת נביאים אחרונים חגי זכריה ומלאכי שהיו ממתקני תקנות ישראל באנשי כנסת הגדולה:
חביט חביט - לשון ניענוע:
He waved, he waved - an expression for "shaking."
א"ל אביי לרבא מאי שנא לולב דעבדינן ליה שבעה זכר למקדש ומאי שנא ערבה דלא עבדינן לה שבעה זכר למקדש א"ל הואיל ואדם יוצא ידי חובתו בערבה שבלולב א"ל ההוא משום לולב הוא דקא עביד ליה וכי תימא דקא מגבה ליה והדר מגבה ליה והא מעשים בכל יום דלא קא עבדינן הכי
Abaye said to Rava: What is different about lulav such that we perform the mitzva seven days in commemoration of the Temple, and what is different about the willow branch that we do not perform the mitzva seven days in commemoration of the Temple? Rava said to him: Since a person fulfills his obligation with the willow branch in the lulav, no additional commemoration is necessary. Abaye said to him: That is not a satisfactory answer, as he is performing that action due to the mitzva of taking the lulav and the other species. And if you say that he lifts the willow branch bound with the lulav to fulfill the mitzva of the four species and then lifts it again in commemoration of the willow branch in the Temple, aren’t actions performed daily proof that we do not do so, as no one lifts the lulav twice?

R. Pesach Wolicki

In his commentary to Genesis 12:6, Nachmanides writes:
“Know, that any ‘decree of the watchers’ (i.e. prophecy, see Daniel 4:14) when it emerges from the force of a representative act (poel dimyon) the decree must absolutely be fulfilled. For this reason, prophets perform prophetic actions such as Jeremiah’s command to Baruch to tie a stone to the scroll and to throw it in the river and to say ‘so will Bavel sink.’ (Jeremiah 51:63-64), so too when Elisha placed his arm on the bow… (Kings 13:17).

There are numerous examples of prophets performing actions that represent the content of their prophecy. Nachmanides explains that such actions serve to ratify those prophecies and make them absolutely certain to be fulfilled. (The implications of this idea for prophecies that do not involve such actions is discussed at length by the Rabbenu Nissim in Derashot HaRa”N, derasha 2)

Think of the sound of a synagogue full of aravot hitting the floor. The sound is almost identical to that of a hard rainfall. The aravot — willows of the brook — which require abundant water hitting the ground like rain and making the sound of rain. We perform this representative action and say “Kol mevaser!” “The sound heralds!” After many paragraphs of pleading with G-d to save us “Hosha Na!” we take our prayers to another level. Just as a prophet ratifies and secures the positive outcome of his prophecy through representative action, we make our prayers more likely to be fulfilled by performing an action that represents those prayers. It is no longer merely a request. We have done a representative action. In the words of the Nachmanides, “the decree must absolutely be fulfilled.”

To put it another way: When we hit beat the aravot on the floor we behave like prophets. Minhag neviim — a custom of the prophets — indeed!

Mr. Steven Weiner

Others have suggested that חיבוט ערבות represents a prayer for rain, the sound of beatingערבות evoking the sounds and sights of a rainstorm.[6] This seems plausible, as the Talmud and Midrash indicate that arba minim and other practices of Sukkot are in part connected to our prayers for rain, which begin at this time of year. However, once again this explanation fails to shed light on why the custom was initiated specifically by the prophets of שיבת ציון, or how we can possibly reconcile the seemingly self-contradictory, dual status of מנהג נביאים and זכר למקדש.

(יח) וַיֹּ֛אמֶר קַ֥ח הַחִצִּ֖ים וַיִּקָּ֑ח וַיֹּ֤אמֶר לְמֶֽלֶךְ־יִשְׂרָאֵל֙ הַךְ־אַ֔רְצָה וַיַּ֥ךְ שָֽׁלֹשׁ־פְּעָמִ֖ים וַֽיַּעֲמֹֽד׃ (יט) וַיִּקְצֹ֨ף עָלָ֜יו אִ֣ישׁ הָאֱלֹקִ֗ים וַיֹּ֙אמֶר֙ לְהַכּ֨וֹת חָמֵ֤שׁ אוֹ־שֵׁשׁ֙ פְּעָמִ֔ים אָ֛ז הִכִּ֥יתָ אֶת־אֲרָ֖ם עַד־כַּלֵּ֑ה וְעַתָּ֕ה שָׁלֹ֥שׁ פְּעָמִ֖ים תַּכֶּ֥ה אֶת־אֲרָֽם׃ (ס)
(18) He said, “Now pick up the arrows.” And he picked them up. “Strike the ground!” he said to the king of Israel; and he struck three times and stopped. (19) The man of God was angry with him and said to him, “If only you had struck five or six times! Then you would have annihilated Aram; as it is, you shall defeat Aram only three times.”
(א) בַּשְּׁבִיעִ֕י בְּעֶשְׂרִ֥ים וְאֶחָ֖ד לַחֹ֑דֶשׁ הָיָה֙ דְּבַר־ה' בְּיַד־חַגַּ֥י הַנָּבִ֖יא לֵאמֹֽר׃ (ב) אֱמָר־נָ֗א אֶל־זְרֻבָּבֶ֤ל בֶּן־שַׁלְתִּיאֵל֙ פַּחַ֣ת יְהוּדָ֔ה וְאֶל־יְהוֹשֻׁ֥עַ בֶּן־יְהוֹצָדָ֖ק הַכֹּהֵ֣ן הַגָּד֑וֹל וְאֶל־שְׁאֵרִ֥ית הָעָ֖ם לֵאמֹֽר׃ (ג) מִ֤י בָכֶם֙ הַנִּשְׁאָ֔ר אֲשֶׁ֤ר רָאָה֙ אֶת־הַבַּ֣יִת הַזֶּ֔ה בִּכְבוֹד֖וֹ הָרִאשׁ֑וֹן וּמָ֨ה אַתֶּ֜ם רֹאִ֤ים אֹתוֹ֙ עַ֔תָּה הֲל֥וֹא כָמֹ֛הוּ כְּאַ֖יִן בְּעֵינֵיכֶֽם׃ (ד) וְעַתָּ֣ה חֲזַ֣ק זְרֻבָּבֶ֣ל ׀ נְאֻם־ה' וַחֲזַ֣ק יְהוֹשֻׁ֣עַ בֶּן־יְהוֹצָדָק֩ הַכֹּהֵ֨ן הַגָּד֜וֹל וַחֲזַ֨ק כָּל־עַ֥ם הָאָ֛רֶץ נְאֻם־ה' וַֽעֲשׂ֑וּ כִּֽי־אֲנִ֣י אִתְּכֶ֔ם נְאֻ֖ם ה' צְבָאֽוֹת׃ (ה) אֶֽת־הַדָּבָ֞ר אֲשֶׁר־כָּרַ֤תִּי אִתְּכֶם֙ בְּצֵאתְכֶ֣ם מִמִּצְרַ֔יִם וְרוּחִ֖י עֹמֶ֣דֶת בְּתוֹכְכֶ֑ם אַל־תִּירָֽאוּ׃ (ס) (ו) כִּ֣י כֹ֤ה אָמַר֙ ה' צְבָא֔וֹת ע֥וֹד אַחַ֖ת מְעַ֣ט הִ֑יא וַאֲנִ֗י מַרְעִישׁ֙ אֶת־הַשָּׁמַ֣יִם וְאֶת־הָאָ֔רֶץ וְאֶת־הַיָּ֖ם וְאֶת־הֶחָרָבָֽה׃ (ז) וְהִרְעַשְׁתִּי֙ אֶת־כָּל־הַגּוֹיִ֔ם וּבָ֖אוּ חֶמְדַּ֣ת כָּל־הַגּוֹיִ֑ם וּמִלֵּאתִ֞י אֶת־הַבַּ֤יִת הַזֶּה֙ כָּב֔וֹד אָמַ֖ר ה' צְבָאֽוֹת׃ (ח) לִ֥י הַכֶּ֖סֶף וְלִ֣י הַזָּהָ֑ב נְאֻ֖ם ה' צְבָאֽוֹת׃ (ט) גָּד֣וֹל יִֽהְיֶ֡ה כְּבוֹד֩ הַבַּ֨יִת הַזֶּ֤ה הָאַֽחֲרוֹן֙ מִן־הָ֣רִאשׁ֔וֹן אָמַ֖ר ה' צְבָא֑וֹת וּבַמָּק֤וֹם הַזֶּה֙ אֶתֵּ֣ן שָׁל֔וֹם נְאֻ֖ם ה' צְבָאֽוֹת׃ (פ)
(1) on the twenty-first day of the seventh month, the word of the LORD came through the prophet Haggai: (2) Speak to Zerubbabel son of Shealtiel, the governor of Judah, and to the high priest Joshua son of Jehozadak, and to the rest of the people: (3) Who is there left among you who saw this House in its former splendor? How does it look to you now? It must seem like nothing to you. (4) But be strong, O Zerubbabel—says the LORD—be strong, O high priest Joshua son of Jehozadak; be strong, all you people of the land—says the LORD—and act! For I am with you—says the LORD of Hosts. (5) So I promised you when you came out of Egypt, and My spirit is still in your midst. Fear not! (6) For thus said the LORD of Hosts: In just a little while longer I will shake the heavens and the earth, the sea and the dry land; (7) I will shake all the nations. And the precious things of all the nations shall come [here], and I will fill this House with glory, said the LORD of Hosts. (8) Silver is Mine and gold is Mine—says the LORD of Hosts. (9) The glory of this latter House shall be greater than that of the former one, said the LORD of Hosts; and in this place I will grant prosperity—declares the LORD of Hosts.
(א) וַיָּ֕שָׁב הַמַּלְאָ֖ךְ הַדֹּבֵ֣ר בִּ֑י וַיְעִירֵ֕נִי כְּאִ֖ישׁ אֲשֶׁר־יֵע֥וֹר מִשְּׁנָתֽוֹ׃ (ב) וַיֹּ֣אמֶר אֵלַ֔י מָ֥ה אַתָּ֖ה רֹאֶ֑ה ויאמר [וָאֹמַ֡ר] רָאִ֣יתִי ׀ וְהִנֵּ֣ה מְנוֹרַת֩ זָהָ֨ב כֻּלָּ֜הּ וְגֻלָּ֣הּ עַל־רֹאשָׁ֗הּ וְשִׁבְעָ֤ה נֵרֹתֶ֙יהָ֙ עָלֶ֔יהָ שִׁבְעָ֤ה וְשִׁבְעָה֙ מֽוּצָק֔וֹת לַנֵּר֖וֹת אֲשֶׁ֥ר עַל־רֹאשָֽׁהּ׃ (ג) וּשְׁנַ֥יִם זֵיתִ֖ים עָלֶ֑יהָ אֶחָד֙ מִימִ֣ין הַגֻּלָּ֔ה וְאֶחָ֖ד עַל־שְׂמֹאלָֽהּ׃ (ד) וָאַ֙עַן֙ וָֽאֹמַ֔ר אֶל־הַמַּלְאָ֛ךְ הַדֹּבֵ֥ר בִּ֖י לֵאמֹ֑ר מָה־אֵ֖לֶּה אֲדֹנִֽי׃ (ה) וַ֠יַּעַן הַמַּלְאָ֞ךְ הַדֹּבֵ֥ר בִּי֙ וַיֹּ֣אמֶר אֵלַ֔י הֲל֥וֹא יָדַ֖עְתָּ מָה־הֵ֣מָּה אֵ֑לֶּה וָאֹמַ֖ר לֹ֥א אֲדֹנִֽי׃ (ו) וַיַּ֜עַן וַיֹּ֤אמֶר אֵלַי֙ לֵאמֹ֔ר זֶ֚ה דְּבַר־ה' אֶל־זְרֻבָּבֶ֖ל לֵאמֹ֑ר לֹ֤א בְחַ֙יִל֙ וְלֹ֣א בְכֹ֔חַ כִּ֣י אִם־בְּרוּחִ֔י אָמַ֖ר ה' צְבָאֽוֹת׃ (ז) מִֽי־אַתָּ֧ה הַֽר־הַגָּד֛וֹל לִפְנֵ֥י זְרֻבָּבֶ֖ל לְמִישֹׁ֑ר וְהוֹצִיא֙ אֶת־הָאֶ֣בֶן הָרֹאשָׁ֔ה תְּשֻׁא֕וֹת חֵ֥ן חֵ֖ן לָֽהּ׃ (פ) (ח) וַיְהִ֥י דְבַר־ה' אֵלַ֥י לֵאמֹֽר׃ (ט) יְדֵ֣י זְרֻבָּבֶ֗ל יִסְּד֛וּ הַבַּ֥יִת הַזֶּ֖ה וְיָדָ֣יו תְּבַצַּ֑עְנָה וְיָ֣דַעְתָּ֔ כִּֽי־ה' צְבָא֖וֹת שְׁלָחַ֥נִי אֲלֵיכֶֽם׃ (י) כִּ֣י מִ֣י בַז֮ לְי֣וֹם קְטַנּוֹת֒ וְשָׂמְח֗וּ וְרָא֞וּ אֶת־הָאֶ֧בֶן הַבְּדִ֛יל בְּיַ֥ד זְרֻבָּבֶ֖ל שִׁבְעָה־אֵ֑לֶּה עֵינֵ֣י ה' הֵ֥מָּה מְשׁוֹטְטִ֖ים בְּכָל־הָאָֽרֶץ׃
(1) The angel who talked with me came back and woke me as a man is wakened from sleep. (2) He said to me, “What do you see?” And I answered, “I see a lampstand all of gold, with a bowl above it. The lamps on it are seven in number, and the lamps above it have seven pipes; (3) and by it are two olive trees, one on the right of the bowl and one on its left.” (4) I, in turn, asked the angel who talked with me, “What do those things mean, my lord?” (5) “Do you not know what those things mean?” asked the angel who talked with me; and I said, “No, my lord.” (6) Then he explained to me as follows: “This is the word of the LORD to Zerubbabel: Not by might, nor by power, but by My spirit—said the LORD of Hosts. (7) Whoever you are, O great mountain in the path of Zerubbabel, turn into level ground! For he shall produce that excellent stone; it shall be greeted with shouts of ‘Beautiful! Beautiful!’” (8) And the word of the LORD came to me: (9) “Zerubbabel’s hands have founded this House and Zerubbabel’s hands shall complete it. Then you shall know that it was the LORD of Hosts who sent me to you. (10) Does anyone scorn a day of small beginnings? When they see the stone of distinction in the hand of Zerubbabel, they shall rejoice. “Those seven are the eyes of the LORD, ranging over the whole earth.”