Deuteronomy 20:16-18

But leave nothing alive in the cities of the land the LORD your God is giving you. Completely destroy these people: the Hittites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites, as the LORD your God has commanded. Otherwise, they will teach you what they do for their gods, and if you do these hateful things, you will sin against the LORD your God.