You shall not add to the word which I command you, nor take from it, that you may keep the commandments of the LORD your God which I command you.
Deuteronomy 4:2
Surrounding context
4:1 "Now, O Israel, listen to the statutes and the judgments which I teach you to observe, that you may live, and go in and possess the land which the LORD God of your fathers is giving you.
4:2 You shall not add to the word which I command you, nor take from it, that you may keep the commandments of the LORD your God which I command you.
4:3 Your eyes have seen what the LORD did at Baal Peor; for the LORD your God has destroyed from among you all the men who followed Baal of Peor.