Numbers 8:13-15

And thou shalt set the Levites before Aaron, and before his sons, and offer them for a wave-offering unto Jehovah. Thus shalt thou separate the Levites from among the children of Israel; and the Levites shall be mine. And after that shall the Levites go in to do the service of the tent of meeting: and thou shalt cleanse them, and offer them for a wave-offering.