In the side, i.e. in the outside, in a little chest fixed to it, for nothing but the tables of stone were contained in the ark, 1 Kings 8:9. Here it was kept for greater security and reverence. A witness against thee, i.e. against thy people, to whom he turns his speech, that they might be more affected with it.
Related Commentaries of Deuteronomy 31:26
Deuteronomy 31:26
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.