Deuteronomy 4:2

4:2 Do not add a thing to what I command you nor subtract from it, so that you may keep the commandments of the Lord your God that I am delivering to you.

Deuteronomy 19:9

19:9 and then you are careful to observe all these commandments I am giving you today (namely, to love the Lord your God and to always walk in his ways), then you must add three more cities to these three.