Numbers 31:15-18

15Moses said to them, ‘Why have you let all the women live?

16You should not have done that. These women caused the Israelites to turn against the Lord. They did what Balaam said to them at Peor. As a result, a bad disease killed many of the Lord's people.

17Because of that, kill all the boys, and kill every woman who has had sex with a man.

18But as for the girls who have not had sex with a man, you may keep them for yourselves.’