Deuteronomy 20:16-18

In those towns that the LORD your God is giving you as a special possession, destroy every living thing. You must completely destroy the Hittites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites, just as the LORD your God has commanded you. This will prevent the people of the land from teaching you to imitate their detestable customs in the worship of their gods, which would cause you to sin deeply against the LORD your God.