Numbers 8:11-14

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. 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. You are to have the Levites stand before Aaron and his sons, and then offer them as a wave offering to the LORD. And so you are to separate the Levites from among the Israelites, and the Levites will be mine.