אָ֣ז יָשִֽׁיר־מֹשֶׁה֩ וּבְנֵ֨י יִשְׂרָאֵ֜ל אֶת־הַשִּׁירָ֤ה הַזֹּאת֙ לַֽיהוה וַיֹּאמְר֖וּ לֵאמֹ֑ר אָשִׁ֤ירָה לַֽיהוה כִּֽי־גָאֹ֣ה גָּאָ֔ה ס֥וּס וְרֹכְב֖וֹ רָמָ֥ה בַיָּֽם׃ עׇזִּ֤י וְזִמְרָת֙ יָ֔הּ וַֽיְהִי־לִ֖י לִֽישׁוּעָ֑ה זֶ֤ה אֵלִי֙ וְאַנְוֵ֔הוּ אֱלֹהֵ֥י אָבִ֖י וַאֲרֹמְמֶֽנְהוּ׃ יהוה אִ֣ישׁ מִלְחָמָ֑ה יהוה שְׁמֽוֹ׃ מַרְכְּבֹ֥ת פַּרְעֹ֛ה וְחֵיל֖וֹ יָרָ֣ה בַיָּ֑ם וּמִבְחַ֥ר שָֽׁלִשָׁ֖יו טֻבְּע֥וּ בְיַם־סֽוּף׃ תְּהֹמֹ֖ת יְכַסְיֻ֑מוּ יָרְד֥וּ בִמְצוֹלֹ֖ת כְּמוֹ־אָֽבֶן׃
Then Moses and the Israelites sang this song to יהוה. They said: I will sing to יהוה, for He has triumphed gloriously; Horse and driver He has hurled into the sea. יהוה is my strength and might; He is become my deliverance. This is my God and I will enshrine Him; The God of my father’s [house], and I will exalt Him. יהוה, the Warrior— יהוה is His name! Pharaoh’s chariots and his army He has cast into the sea; And the pick of his officers Are drowned in the Sea of Reeds. The deeps covered them; They went down into the depths like a stone.
אָ֣ז יָשִֽׁיר־מֹשֶׁה֩ - Then sang Moses
With regard to the usage of the future ישיר, yashir, the meaning is: THEN — i. e. when he saw the miracle it entered his mind that HE WOULD SING a song…. So, also, ישיר yashir here signifies: his heart told him that he should sing, and thus did he actually do, as it states, “and they (Moses and Israel) spoke as follows, ‘I will sing unto the Lord’”....
The word אז, az, denotes only joy, as it is written, “Then [אז, az] our mouths will be filled with laughter [Ps. 126:2].”
וַיֹּאמְר֖וּ לֵאמֹ֑ר - And said, saying
First we must understand that the essence of joy is what a person feels in his heart; seeing that this is so, what need is there to express these feelings in speech and song or poetry at the time of his joy? The reason is that joy in one’s heart, unless formulated in word and song is bound to subside and cease altogether in short order. By giving verbal expression to one’s joy and composing a song and writing poetry one prolongs and intensifies this feeling of joy. The Israelites were aware of this psychological axiom, and this is why they yearned to give proper expression to their joy. They yearned to share their joy with the Creator, Who had been the architect enabling them to harbor such joyous feelings in their breasts. This is the reason why the Torah adds the words: ויאמרו לאמור, va’yomru l’emor, “they said, saying;” the word לאמור, l’emor, presumably refers to extraneous words, not included in the actual song that follows; if this were not so, who was there that they could have spoken to about this other than their peers who had all experienced the same salvation?
Why do we read this on Passover?
Rabbi Tzaddok HaKohein of Lublin, Sefer P’ri Tzaddik
The intent of the Haggadah is not just the story itself...one is obliged to tell the story in such a way that he himself feels, and makes others feel, that they are indeed in the process of leaving Egypt and getting ready to go through the Sea. One must feel the joy and the freedom in his soul as if he is really leaving Egypt for good.