Deuteronomy 20:16-18

But of the cities of these people, which the LORD thy God doth give thee for an inheritance, thou shalt save alive nothing that breatheth: but thou shalt utterly destroy them; namely, the Hittites, and the Amorites, the Canaanites, and the Perizzites, the Hivites, and the Jebusites; as the LORD thy God hath commanded thee: that they teach you not to do after all their abominations, which they have done unto their gods; so should ye sin against the LORD your God.