Numbers 8:11-14

and Aaron shall offer the Levites before the LORD like a wave offering from the children of Israel, that they may perform the work of the LORD. Then the Levites shall lay their hands on the heads of the young bulls, and you shall offer one as a sin offering and the other as a burnt offering to the LORD, to make atonement for the Levites. “And you shall stand the Levites before Aaron and his sons, and then offer them like a wave offering to the LORD. Thus you shall separate the Levites from among the children of Israel, and the Levites shall be Mine.