Deuteronomy 20:15-18

15Thus shalt thou do to all the cities which are very distant from thee, which are not of the cities of these nations. 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.