Ramban on Deuteronomy 31:19:1

וְעַתָּה כִּתְבוּ לָכֶם אֶת הַשִּׁירָה הַזֹּאת, עַל דֶּרֶךְ הַפְּשָׁט לְמֹשֶׁה וִיהוֹשֻׁעַ שְׁנֵיהֶם יְצַוֶּה שֶׁיִּכְתְּבוּהָ, כִּי רָצָה לַעֲשׂוֹת יְהוֹשֻׁעַ נְבִיאוֹ בְּחַיֵּי מֹשֶׁה, וְהִנֵּה מֹשֶׁה כְּתָבָהּ וִיהוֹשֻׁעַ עוֹמֵד עִמּוֹ וְקוֹרֵא וְרוֹאֶה, וְזֶה טַעַם וַיִּכְתֹּב מֹשֶׁה (דברים ל"א:כ"ב). וְלַמְּדָהּ אֶת בְּנֵי יִשְׂרָאֵל, לְמֹשֶׁה שֶׁהוּא הָעִקָּר, כִּי מִמֶּנּוּ יִשְׁמְעוּ וְיִלְמְדוּ, כִּי גַּם יְהוֹשֻׁעַ הָיָה מְלַמֵּד עִמּוֹ, כְּמוֹ שֶׁאָמַר בַּסּוֹף (דברים ל"ב:מ"ד) הוּא וְהוֹשֵׁעַ בִּן נוּן. וְטַעַם הַשִּׁירָה הַזֹּאת, הַשִּׁירָה אֲשֶׁר אַגִּיד לְךָ עַתָּה וְהִיא הַאֲזִינוּ, וְיִקְרָאֶהָ שִׁירָה כִּי יִשְׂרָאֵל יֹאמְרוּהָ תָּמִיד בְּשִׁיר וּבְזִמְרָה, וְכֵן נִכְתְּבָה כְּשִׁירָה, כִּי הַשִּׁירִים יִכְתְּבוּ בָּהֶם הֶפְסֵק בִּמְקוֹמוֹת הַנְּעִימָה: AND NOW WRITE YE THIS SONG. In line with the plain meaning of Scripture, He commands both Moses and Joshua to write it for He wanted to make Joshua His prophet during Moses’ lifetime. Now, Moses wrote it while Joshua stood by him, read it, and saw [it completed], this being the sense of the verse, So Moses wrote this Song the same day, and taught it the children of Israel.³⁶Further, Verse 22. It refers to Moses who was the principal [in the writing of the Song] for it was from him that they would hear it and learn it, although Joshua, too, taught it with him, as it is stated at the conclusion [of the chapter] he [i.e., Moses], and Hoshea, the son of Nun.³⁷Further, 32:44.
Now, the meaning of the expression this Song is “the Song that I will now tell you,” this being ‘Ha’azinu’ (Give ear).³⁸Ibid., Verses 1-43. It is called Song because Israel would recite it with song and psalm.³⁹At the Divine Service in the Sanctuary, a section of the Song of Ha’azinu was chanted each Sabbath during the Service of the Additional Offering. The cycle of the Song was completed every six weeks. Therefore, the six divisions (Aliyoth) in the reading of this Song in the Synagogue on the Sabbath of Ha’azinu, follow the same divisions as sung in the Sanctuary (Rosh Hashanah 31a). It was also written [in the Torah] in the style of a song, since songs are written with interruptions [empty spaces] indicating the places of chanting.