Numbers 8:11-15

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.

15After you have purified them and presented them as a wave offering, the Levites will come to do their work at the Tent of Meeting.