C. F. Keil and Franz Delitzsch on Numbers 31:19-20

Num 31:19-20 Purification of the Warriors, the Prisoners, and the Booty. - Moses commanded the men of war to remain for seven days outside the camp of the congregation, to carry out upon the third and seventh day the legal purification of such persons and things as had been rendered unclean through contact with dead bodies. Every one who had slain a soul (person), or touched one who had been slain, was to be purified, whether he were a warrior or a prisoner. And so also were all the clothes, articles of leather, materials of goats’ hair, and all wooden things.