Deuteronomy 20:15-18

Thus you shall do to all the cities which are very far from you, which are not of the cities of these nations. “But of the cities of these peoples which the LORD your God gives you as an inheritance, you shall let nothing that breathes remain alive, but you shall utterly destroy them: the Hittite and the Amorite and the Canaanite and the Perizzite and the Hivite and the Jebusite, just as the LORD your God has commanded you, lest they teach you to do according to all their abominations which they have done for their gods, and you sin against the LORD your God.