Numbers 8:11-14
8:11 and 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.
8:12 When 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.
8:13 You are to have the Levites stand before Aaron and his sons, and then offer them as a wave offering to the Lord.
8:14 And so you are to separate the Levites from among the Israelites, and the Levites will be mine.