Numbers 8:10

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 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.