Deuteronomy 31:19-22

19“Now then, write this song for yourselves, and teach it to the sons of Israel; put it in their mouth, so that this song may be a witness for Me against the sons of Israel.

20For when I bring them into the land which I have sworn to their fathers, a land [of plenty] flowing with milk and honey, and they have eaten and are satisfied and become prosperous, then they will turn to other gods and serve them, and despise and reject Me and break My covenant.

21Then it shall come about, when many evils and troubles have come on them, that this [sacred] song will confront them as a witness; for it will not be forgotten from the mouth of their descendants. For I know their inclination which is developing even now, before I bring them into the land which I have sworn to give them.”

22So Moses wrote this song the same day, and taught it to the children of Israel.