Numbers 8:10-14

10Then you are to bring the Levites before the Lord, and the Israelites are to lay their hands on the Levites;

11and Aaron is to offer the Levites before the Lord as a wave offering from the Israelites, that they may do the work of the Lord.

12When the Levites lay their hands on the heads of the bulls, offer the one for a purification offering and the other for a whole burnt offering to the Lord, to make atonement for the Levites.

13You are to have the Levites stand before Aaron and his sons, and then offer them as a wave offering to the Lord.

14And so you are to separate the Levites from among the Israelites, and the Levites will be mine.