Numbers 31:12 (KJV)

And they warred against the Midianites, as the LORD commanded Moses; and they slew all the males.

And they slew the kings of Midian, beside the rest of them that were slain; namely, Evi, and Rekem, and Zur, and Hur, and Reba, five kings of Midian: Balaam also the son of Beor they slew with the sword.

And the children of Israel took all the women of Midian captives, and their little ones, and took the spoil of all their cattle, and all their flocks, and all their goods.

And they burnt all their cities wherein they dwelt, and all their goodly castles, with fire.

And they took all the spoil, and all the prey, both of men and of beasts.

And Moses, and Eleazar the priest, and all the princes of the congregation, went forth to meet them without the camp.

And Moses was wroth with the officers of the host, with the captains over thousands, and captains over hundreds, which came from the battle.

And Moses said unto them, Have ye saved all the women alive?

Behold, these caused the children of Israel, through the counsel of Balaam, to commit trespass against the LORD in the matter of Peor, and there was a plague among the congregation of the LORD.

Now therefore kill every male among the little ones, and kill every woman that hath known man by lying with him.