Numbers 8:8-12

Then let them take a young bullock with his meat offering, even fine flour mingled with oil, and another young bullock shalt thou take for a sin offering.

And thou shalt bring the Levites before the tabernacle of the congregation: and thou shalt gather the whole assembly of the children of Israel together:

And thou shalt bring the Levites before the LORD: and the children of Israel shall put their hands upon the Levites:

And Aaron shall offer the Levites before the LORD for an offering of the children of Israel, that they may execute the service of the LORD.

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