Numbers 8:8-12

Then have them take a bull with its grain offering, fine flour mixed with oil; and you shall take a second bull as a sin offering.

So you shall present the Levites in front of the tent of meeting. You shall also assemble the whole congregation of the sons of Israel,

and present the Levites before the Lord; and the sons of Israel shall lay their hands on the Levites.

Aaron then shall present the Levites before the Lord as a wave offering from the sons of Israel, so that they may qualify to perform the service of the Lord.

Now the Levites shall lay their hands on the heads of the bulls; then you are to offer the one as a sin offering and the other as a burnt offering to the Lord, to make atonement for the Levites.