Deuteronomy 31:21 in the King James Version
And it shall come to pass, when many evils and troubles are befallen them, that this song shall testify against them as a witness; for it shall not be forgotten out of the mouths of their seed: for I know their imagination which they go about, even now, before I have brought them into the land which I sware.
Context
20 For when I have brought them into the land which I swore to their fathers, flowing with milk and honey, and they have eaten and filled themselves, and grown fat, then they will turn to other gods, and serve them, and despise me, and break my covenant.
21 It will happen, when many evils and troubles have come on them, that this song will testify before them as a witness; for it will not be forgotten out of the mouths of their descendants; for I know their ways and what they are doing today, before I have brought them into the land which I promised them.”
22 So Moses wrote this song the same day, and taught it to the children of Israel.