Deuteronomy 13:15-17

thou shalt surely smite the inhabitants of that city with the edge of the sword, destroying it utterly, and all that is therein and the cattle thereof, with the edge of the sword. And thou shalt gather all the spoil of it into the midst of the street thereof, and shalt burn with fire the city, and all the spoil thereof every whit, unto Jehovah thy God: and it shall be a heap for ever; it shall not be built again. And there shall cleave nought of the devoted thing to thy hand; that Jehovah may turn from the fierceness of his anger, and show thee mercy, and have compassion upon thee, and multiply thee, as he hath sworn unto thy fathers