Mishnayos Sotah Perek 5 Mishnah 4

Additionally, on that same day Rabbi Akiva interpreted the verse: “Then Moses and the children of Israel sang this song to the Lord, and said, saying” (Exodus 15:1), as follows: As there is no need for the verse to state the word “saying,” because it states the word “said” immediately prior to it, why must the verse state the word “saying”? It teaches that the Jewish people would repeat in song after Moses every single statement he said, as is done when reciting hallel. After Moses would recite a verse, they would say as a refrain: “I will sing to the Lord, for He is highly exalted” (Exodus 15:1). It is for this reason that the word “saying” is stated, in addition to the word “said.” Rabbi Neḥemya says: The people sang the song together with Moses as is done when reciting Shema, which is recited in unison after the prayer leader begins, and not as is done when reciting hallel.

בּוֹ בַיּוֹם דָּרַשׁ רַבִּי עֲקִיבָא (שמות טו), אָז יָשִׁיר משֶׁה וּבְנֵי יִשְׂרָאֵל אֶת הַשִּׁירָה הַזֹּאת לַה' וַיֹּאמְרוּ לֵאמֹר, שֶׁאֵין תַּלְמוּד לוֹמַר לֵאמֹר, וּמַה תַּלְמוּד לוֹמַר לֵאמֹר, מְלַמֵּד שֶׁהָיוּ יִשְׂרָאֵל עוֹנִין אַחֲרָיו שֶׁל משֶׁה עַל כָּל דָּבָר וְדָבָר, כְּקוֹרִין אֶת הַהַלֵּל, לְכָךְ נֶאֱמַר לֵאמֹר. רַבִּי נְחֶמְיָה אוֹמֵר, כְּקוֹרִין אֶת שְׁמַע וְלֹא כְקוֹרִין אֶת הַהַלֵּל: