Deuteronomy 20:16-18

But of the cities of these peoples, that Jehovah thy God giveth thee for an inheritance, thou shalt save alive nothing that breatheth; but thou shalt utterly destroy them: the Hittite, and the Amorite, the Canaanite, and the Perizzite, the Hivite, and the Jebusite; as Jehovah thy God hath commanded thee; that they teach you not to do after all their abominations, which they have done unto their gods; so would ye sin against Jehovah your God.