Numbers 8:11-15

11Aaron shall present the Levites before the Lord as a wave offering from the Israelites, so that they may perform the service of the Lord.

12Then 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.

13You shall have the Levites stand before Aaron and his sons and present them as a wave offering to the Lord.

14“Thus you shall separate the Levites from among the Israelites, and the Levites shall be Mine [in a very special sense].

15Then after that the Levites may go in to serve the Tent of Meeting (tabernacle), but you shall cleanse them [first] and present them as a wave offering;