Deuteronomy 31:26
"Take this book of the law, and put it in the side of the ark of the covenant of the LORD your God, that it may be there for a witness against thee."
Cross References (ranked by relevance)
Deuteronomy 31:19
Now therefore write ye this song for you, and teach it the children of Israel: put it in their mouths, that this song may be a witness for me against the children of Israel.
1 Kings 8:9
There was nothing in the ark save the two tables of stone, which Moses put there at Horeb, when the LORD made a covenant with the children of Israel, when they came out of the land of Egypt.
Galatians 2:19
For I through the law am dead to the law, that I might live unto God.