Numbers 8:12-14

“Then the Levites are to lay their hands on the heads of the bulls, using one for a sin offering to the LORD and the other for a burnt offering, to make atonement for the Levites. Have the Levites stand in front of Aaron and his sons and then present them as a wave offering to the LORD. In this way you are to set the Levites apart from the other Israelites, and the Levites will be mine.