Numbers 8:12-14

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.