Numbers 8:14-16

“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; for they are wholly given to Me from among the Israelites. I have taken them for Myself instead of all who are born first, the firstborn of all the Israelites.