This is printed after Aleinu in many siddurim. I haven't heard it said aloud in most Ashkenazi shuls I've been in, but Chabad congregations tend to sing it.
אַל תִּירָא מִפַּחַד פִּתאֹם וּמִשֹׁאַת רְשָׁעִים כִּי תָבֹא עֻֽצוּ עֵצָה וְתֻפָר דַּבְּ֒רוּ דָבָר וְלֹא יָקוּם כִּי עִמָּנוּ אֵל: וְעַד זִקְנָה אֲנִי הוּא וְעַד שֵׂיבָה אֲנִי אֶסְבֹּל: אֲנִי עָשִֽׂיתִי וַאֲנִי אֶשָׂא וַאֲנִי אֶסְבֹּל וַאֲמַלֵּט:
Do not fear sudden terror nor the destruction of the wicked when it comes take counsel together but it will be foiled speak the word and it will not stand for God is with us. To Your old age I am [with You], to Your older years I will sustain You, I have made You, and I will carry You, I will sustain You and save You.
I've been saying it after Aleinu since some point in the aftermath of October 7th, 2023, with additional intent since some point in the aftermath of January 20th, 2025.
It is three verses from Mishlei and Yesha'yahu, strung together thanks to this story, found in the Midrash - Esther Rabba 7:17 (not numbered correctly in my edition and Sefaria's):
וּבְשָׁעָה שֶׁנֶּחְתְּמוּ אוֹתָן הָאִגְּרוֹת וְנִתְּנוּ בְּיַד הָמָן, וַיָּבוֹא שָׂמֵחַ הוּא וְכָל בְּנֵי חֲבוּרָתוֹ, וּפָגְעוּ בְּמָרְדֳּכַי שֶׁהוּא הוֹלֵךְ לִפְנֵיהֶם, וְרָאָה מָרְדֳּכַי שְׁלשָׁה תִּינוֹקוֹת שֶׁהָיוּ בָּאִים מִבֵּית הַסֵּפֶר, וְרָץ מָרְדֳּכַי אַחֲרֵיהֶם, וּכְשֶׁרָאָה הָמָן וְכָל חֲבוּרָתוֹ שֶׁהָיָה רָץ מָרְדֳּכַי אַחֲרֵי הַתִּינוֹקוֹת, הָלְכוּ אַחֲרֵי מָרְדֳּכַי לָדַעַת מַה יִּשְׁאַל מָרְדֳּכַי מֵהֶם, כֵּיוָן שֶׁהִגִּיעַ מָרְדֳּכַי אֵצֶל הַתִּינוֹקוֹת שָׁאַל לְאֶחָד מֵהֶם פְּסֹק לִי פְּסוּקֶיךָ, אָמַר לוֹ (משלי ג, כה): אַל תִּירָא מִפַּחַד פִּתְאֹם וּמִשֹּׁאַת רְשָׁעִים כִּי תָבֹא. פָּתַח הַשֵּׁנִי וְאָמַר, אֲנִי קָרִיתִי הַיּוֹם וּבָזֶה הַפָּסוּק עָמַדְתִּי מִבֵּית הַסֵּפֶר (ישעיה ח, י): עֻצוּ עֵצָה וְתֻפָר דַּבְּרוּ דָבָר וְלֹא יָקוּם כִּי עִמָּנוּ אֵל. פָּתַח הַשְּׁלִישִׁי וְאָמַר (ישעיה מו, ד): וְעַד זִקְנָה אֲנִי הוּא וְעַד שֵׂיבָה אֲנִי אֶסְבֹּל אֲנִי עָשִׂיתִי וַאֲנִי אֶשָֹּׂא וַאֲנִי אֶסְבֹּל וַאֲמַלֵּט. כֵּיוָן שֶׁשָּׁמַע מָרְדֳּכַי כָּךְ שָׂחַק וְהָיָה שָׂמֵחַ שִׂמְחָה גְדוֹלָה. אָמַר לוֹ הָמָן מָה הִיא זֹאת הַשִֹּׂמְחָה שֶׁשָֹּׂמַחְתָּ לְדִבְרֵי הַתִּינוֹקוֹת הַלָּלוּ, אָמַר עַל בְּשׂוֹרוֹת טוֹבוֹת שֶׁבִּשְׂרוּנִי שֶׁלֹא אֶפְחַד מִן הָעֵצָה הָרָעָה שֶׁיָּעַצְתָּ עָלֵינוּ. מִיָּד כָּעַס הָמָן הָרָשָׁע וְאָמַר אֵין אֲנִי שׁוֹלֵחַ יָדַי תְּחִלָּה אֶלָּא בְּאֵלּוּ הַתִּינוֹקוֹת.
When those letters were sealed and given to Haman, he and all the members of his entourage proceeded joyfully. They met Mordekhai, who was walking ahead of them. Mordekhai saw three children who were coming from school and Mordekhai ran after them. When Haman and his entire entourage saw that Mordekhai was running after the children, they followed Mordekhai to ascertain what Mordekhai would ask them. When Mordekhai reached the children, he asked one of them: ‘Recite your verse to me.’ He said to him: ‘“Fear not from sudden terror, and from the cataclysm of the wicked when it comes”’ (Proverbs 3:25). The second began and said: ‘I read today and with this verse I left school: “Take counsel and it will be negated; speak something and it will not stand, as God is with us”’ (Isaiah 8:10). The third began and said: ‘“Until old age I am He and until gray hairs I will bear you; I have done and I will carry; I will bear and I will rescue”’ (Isaiah 46:4). When Mordekhai heard this, he laughed and was overjoyed. Haman said to him: ‘What is this joy that you rejoiced when hearing the words of these children?’ He said: ‘It is over the good tidings of which they apprised me, that I need not fear the evil designs that you counseled against us.’ Immediately, the wicked Haman became angry and said: ‘I will strike at none other than these children first.’
Let's take a look at these verses in some context. First, from Mishlei:
(כא) בְּ֭נִי אַל־יָלֻ֣זוּ מֵעֵינֶ֑יךָ נְצֹ֥ר תֻּ֝שִׁיָּ֗ה וּמְזִמָּֽה׃ (כב) וְיִֽהְי֣וּ חַיִּ֣ים לְנַפְשֶׁ֑ךָ וְ֝חֵ֗ן לְגַרְגְּרֹתֶֽיךָ׃ (כג) אָ֤ז תֵּלֵ֣ךְ לָבֶ֣טַח דַּרְכֶּ֑ךָ וְ֝רַגְלְךָ֗ לֹ֣א תִגּֽוֹף׃ (כד) אִם־תִּשְׁכַּ֥ב לֹֽא־תִפְחָ֑ד וְ֝שָׁכַבְתָּ֗ וְֽעָרְבָ֥ה שְׁנָתֶֽךָ׃ (כה) אַל־תִּ֭ירָא מִפַּ֣חַד פִּתְאֹ֑ם וּמִשֹּׁאַ֥ת רְ֝שָׁעִ֗ים כִּ֣י תָבֹֽא׃ (כו) כִּֽי־יהוה יִֽהְיֶ֣ה בְכִסְלֶ֑ךָ וְשָׁמַ֖ר רַגְלְךָ֣ מִלָּֽכֶד׃
(21) My son, do not lose sight of them; Hold on to resourcefulness and foresight. (22) They will give life to your spirit And grace to your throat. (23) Then you will go your way safely And not injure your feet. (24) When you lie down you will be unafraid; You will lie down and your sleep will be sweet. (25) You will not fear sudden terror Or the disaster that comes upon the wicked, (26) For the LORD will be your trust; He will keep your feet from being caught.
(ב) כי תבא. כאשר תבא חשכת הרשעים לא תירא מזה כי לא תגע בך לרעה:
that comes. When the darkness for the wicked comes, don't be afraid of that, for it won't touch harm you. (IM translation)
On to the first verse from Yesha'yahu:
(ט) רֹ֤עוּ עַמִּים֙ וָחֹ֔תּוּ וְהַֽאֲזִ֔ינוּ כֹּ֖ל מֶרְחַקֵּי־אָ֑רֶץ הִתְאַזְּר֣וּ וָחֹ֔תּוּ הִֽתְאַזְּר֖וּ וָחֹֽתּוּ׃ (י) עֻ֥צוּ עֵצָ֖ה וְתֻפָ֑ר דַּבְּר֤וּ דָבָר֙ וְלֹ֣א יָק֔וּם כִּ֥י עִמָּ֖נוּ אֵֽל׃ {ס}
(9) Band together, O peoples—you shall be broken! Listen to this, you remotest parts of the earth: Gird yourselves—you shall be broken; Gird yourselves—you shall be broken! (10) Hatch a plot—it shall be foiled; Agree on action—it shall not succeed. For with us is God!
וחתו. פעל עמד, השברו והאזינו כל מרחקי ארץ כל העמים שבאתם ממרחק ארץ לירושלם האזינו זאת הנבואה כי בתחילה תשברו ואמר כך תשברו:
וחתו is a simple verb: "You will break, and then listen, you remotest parts of the earth" All you nations that came from distant lands to Jerusalem, listen to this prophecy. You will start by breaking [other nations], but ultimately, you will be broken. (IM translation)
And finally:
(ג) שִׁמְע֤וּ אֵלַי֙ בֵּ֣ית יַעֲקֹ֔ב וְכׇל־שְׁאֵרִ֖ית בֵּ֣ית יִשְׂרָאֵ֑ל הַֽעֲמֻסִים֙ מִנִּי־בֶ֔טֶן הַנְּשֻׂאִ֖ים מִנִּי־רָֽחַם׃ (ד) וְעַד־זִקְנָה֙ אֲנִ֣י ה֔וּא וְעַד־שֵׂיבָ֖ה אֲנִ֣י אֶסְבֹּ֑ל אֲנִ֤י עָשִׂ֙יתִי֙ וַאֲנִ֣י אֶשָּׂ֔א וַאֲנִ֥י אֶסְבֹּ֖ל וַאֲמַלֵּֽט׃ {ס}
(3) Listen to Me, O House of Jacob, All that are left of the House of Israel, Who have been carried since birth, Supported since leaving the womb: (4) Till you grow old, I will still be the same; When you turn gray, it is I who will carry; I was the Maker, and I will be the Bearer; And I will carry and rescue [you].
(א) שמעו. אמר וכל שארית על גלות בבל שהיו שארית עשרת השבטים שגלו ראשונה והם לא שבו בשוב גלות בבל: (ב)
Listen He says, "And those remaining from the exile in Bavel, i.e. the remainder of the ten tribes who were exiled first and who did not return with the return from the exile in Bavel.