Deuteronomy 20:15-18

Thus you shall do to all the cities that are very far from you, which are not of the cities of these nations nearby. Only in the cities of these peoples that the LORD your God is giving you as an inheritance, you shall not leave alive anything that breathes. But you shall utterly destroy them, the Hittite and the Amorite, the Canaanite and the Perizzite, the Hivite and the Jebusite, as the LORD your God has commanded you, so that they may not teach you to do according to all their detestable things which they have done for their gods, so that you would sin against the LORD your God.