Deuteronomy 20:15-18

Thus shalt thou do unto all the cities which are very far off from thee, which are not of the cities of these nations. 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.