Numbers 8:11-15

11and Aaron shall offer the Levites before the Lord as a wave offering from the people of Israel, that they may do the service of the Lord.

12Then 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.

13And you shall set the Levites before Aaron and his sons, and shall offer them as a wave offering to the Lord.

14"Thus you shall separate the Levites from among the people of Israel, and the Levites shall be mine.

15And after that the Levites shall go in to serve at the tent of meeting, when you have cleansed them and offered them as a wave offering.