Deuteronomy 13:15-16

13:15 you must by all means slaughter the inhabitants of that city with the sword; annihilate with the sword everyone in it, as well as the livestock. 13:16 You must gather all of its plunder into the middle of the plaza and burn the city and all its plunder as a whole burnt offering to the Lord your God. It will be an abandoned ruin forever – it must never be rebuilt again.