Numbers 8:10-14

10Bring the Levites before Adonai, Bnei-Yisrael will lay their hands on the Levites,

11and Aaron will present the Levites before Adonai as a wave offering from Bnei-Yisrael. Then they may go about the work of the service of Adonai.

12“The Levites are to lay their hands on the heads of the bulls. Use one for a sin offering and the other for a burnt offering to Adonai to make atonement for the Levites.

13Have the Levites stand before Aaron and his sons and present them as a wave offering to Adonai.

14In this way you are to set apart the Levites from Bnei-Yisrael to be Mine.