Numbers 31:30

And of the children of Israelis half, of the portion which had been allotted to the congregation that had remained in camp, thou shall take one portion of fifty, or two per cent. of the persons, of the beeves, of the asses, and of the flocks, of all manner of beasts, and give them unto the Levites, which keep the charge of the Tabernacle of the Lord. This special order was intended for only this one case, and was not to be a precedent.