Deuteronomy 20:15-18

Thus you shall do to all the cities that are very far from you, which are not cities of the nations here. But in the cities of these peoples that the LORD your God is giving you for an inheritance, you shall save alive nothing that breathes, but you shall devote them to complete destruction, the Hittites and the Amorites, the Canaanites and the Perizzites, the Hivites and the Jebusites, as the LORD your God has commanded, that they may not teach you to do according to all their abominable practices that they have done for their gods, and so you sin against the LORD your God.