Numbers 31:16-19

¹⁶Behold, they caused the sons of Israel, through the counsel of Balaam, to be unfaithful to the Lord in the matter of Peor, so that the plague took place among the congregation of the Lord! ¹⁷Now therefore, kill every male among the little ones, and kill every woman who has known a man intimately. ¹⁸However, all the girls who have not known a man intimately, keep alive for yourselves. ¹⁹And as for you, camp outside the camp for seven days; whoever has killed a person and whoever has touched anyone killed, purify yourselves, you and your captives, on the third day and on the seventh day.