Deuteronomy 20:16-18

16But of the cities of these people, which the LORD thy God doth give thee for an inheritance, thou shalt save alive nothing that breatheth: 17But 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: 18That they may not teach you to do after all their abominations, which they have done to their gods; so should ye sin against the LORD your God.