Deuteronomy 20:16-18
Laws Concerning War with Canaanite Nations
20:16 As for the cities of these peoples that the Lord your God is going to give you as an inheritance, you must not allow a single living thing to survive.
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.