Deuteronomy 31:23

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.

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.”

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

When Moses had finished writing the words of this law in a book, until they were finished,

Moses commanded the Levites, who bore the ark of the LORD’s covenant, saying,

“Take this book of the law, and put it by the side of the ark of the LORD your God’s covenant, that it may be there for a witness against you.