Deuteronomy 31:21
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.”
World English Bible (Public Domain)
In context
Before · Deuteronomy 31:19–20
19“Now therefore write this song for yourselves, and teach it to the children of Israel. Put it in their mouths, that this song may be a witness for me against the children of Israel. 20For 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.
21It 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.”