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 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.