Deuteronomy 20:17-18

20:17 Instead you must utterly annihilate them – the Hittites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites – just as the Lord your God has commanded you, 20:18 so that they cannot teach you all the abhorrent ways they worship their gods, causing you to sin against the Lord your God.