Deuteronomy 20:16-18

However, in the cities of the nations the LORD your God is giving you as an inheritance, do not leave alive anything that breathes. Completely destroy them – the Hittites, Amorites, Canaanites, Perizzites, Hivites and Jebusites – as the LORD your God has commanded you. Otherwise, they will teach you to follow all the detestable things they do in worshipping their gods, and you will sin against the LORD your God.