Numbers 8:11-15

11And Aaron shall offer the Levites before the LORD for an offering of the children of Israel, that they may execute the service of the LORD.123 12And the Levites shall lay their hands upon the heads of the bulls: and thou shalt offer the one for a sin offering, and the other for a burnt offering, to the LORD, to make an atonement for the Levites. 13And thou shalt set the Levites before Aaron, and before his sons, and offer them for an offering to the LORD. 14Thus shalt thou separate the Levites from among the children of Israel: and the Levites shall be mine. 15And after that shall the Levites go in to perform the service of the tabernacle of the congregation: and thou shalt cleanse them, and offer them for an offering.