Rashi on Deuteronomy 31:21/רש"י על דברים ל״א:כ״א

1וענתה השירה הזאת לפניו לעד [AND IT SHALL COME TO PASS …] THAT THIS SONG SHALL TESTIFY that in it I had warned them concerning all the things that had befallen them.

2כי לא תשכח מפי זרעו FOR IT SHALL NOT BE FORGOTTEN OUT OF THE MOUTHS OF THEIR SEED — This is an assurance to Israel that the Torah in which this song is contained will never be entirely forgotten by their descendants (Shabbat 138b).