Numbers 8:10

8Then let them take a young bull and its meal offering, fine flour mixed with oil; and another young bull you shall take for a sin offering.

9You shall present the Levites before the Tent of Meeting. You shall assemble the whole congregation of the children of Israel.

10You shall present the Levites before the LORD. The children of Israel shall lay their hands on the Levites,

11and Aaron shall offer the Levites before the LORD for a wave offering on the behalf of the children of Israel, that it may be theirs to do the service of the LORD.

12“The Levites shall lay their hands on the heads of the bulls, and you shall offer the one for a sin offering and the other for a burnt offering to the LORD, to make atonement for the Levites.