Numbers 8:14

12“The Levites shall lay their hands on the heads of the bulls, and you shall offer the one for a sin offering and the other for a burnt offering to the LORD, to make atonement for the Levites.

13You shall set the Levites before Aaron and before his sons, and offer them as a wave offering to the LORD.

14Thus you shall separate the Levites from among the children of Israel, and the Levites shall be mine.

15“After that, the Levites shall go in to do the service of the Tent of Meeting. You shall cleanse them, and offer them as a wave offering.

16For they are wholly given to me from among the children of Israel; instead of all who open the womb, even the firstborn of all the children of Israel, I have taken them to me.