Numbers 8:8-12

8Then let them take a young bullock, and its meal offering, fine flour mingled with oil, and another young bullock shalt thou take for a sin offering.

9And thou shalt present the Levites before the tent of meeting: and thou shalt assemble the whole congregation of the children of Israel:

10and thou shalt present the Levites before the LORD: and the children of Israel shall lay their hands upon the Levites:

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

12And the Levites shall lay their hands upon the heads of the bullocks: and offer thou the one for a sin offering, and the other for a burnt offering, unto the LORD, to make atonement for the Levites.