Deuteronomy 20:15-18

That is what you shall do to all the cities that are very far away from you, which are not among the cities of these nations nearby [which you are to dispossess]. 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, the Amorite, the Canaanite, the Perizzite, the Hivite and the Jebusite, just as the LORD your God has commanded you, so that they will not teach you to act in accordance with all the detestable practices which they have done [in worship and service] for their gods, and in this way cause you to sin against the LORD your God.