Numbers 8:13-15

You shall have the Levites stand before Aaron and his sons and present them as a wave offering to the LORD. “Thus you shall separate the Levites from among the Israelites, and the Levites shall be Mine [in a very special sense]. Then after that the Levites may go in to serve the Tent of Meeting (tabernacle), but you shall cleanse them [first] and present them as a wave offering