Numbers 8:10-14

10You are to present the Levites before the LORD and have the Israelites lay their hands upon them. 11Aaron is to present the Levites before the LORD as a wave offering from the sons of Israel, so that they may perform the service of the LORD. 12And the Levites are to lay their hands on the heads of the bulls, and offer to the LORD one as a sin offering and the other as a burnt offering, to make atonement for the Levites.

13You are to have the Levites stand before Aaron and his sons and then present them before the LORD as a wave offering. 14In this way you shall separate the Levites from the rest of the Israelites, and the Levites will belong to Me.