Numbers 31:15-18

15And Moses said to them, “Have you let all the women live?

16Look, these [are the women who] caused the Israelites, by the counsel of Balaam, to trespass against the Lord in the matter of Peor, and so a plague came among the congregation of the Lord.

17Now therefore, kill every male among the children, and kill every woman who is not a virgin.

18But all the young girls who have not known a man intimately, keep alive for yourselves [to marry].