Numbers 8:14KJV

12And the Levites shall lay their hands upon the heads of the bullocks: and thou shalt offer the one a sin offering, and the other a burnt offering, unto the LORD, to make an atonement for the Levites.

13And thou shalt set the Levites before Aaron, and before his sons, and offer them an offering unto the LORD.

14Thus shalt thou separate the Levites from among the children of Israel: and the Levites shall be mine.

15And after that shall the Levites go in to do the service of the tabernacle of the congregation: and thou shalt cleanse them, and offer them an offering.

16For they wholly given unto me from among the children of Israel; instead of such as open every womb, the firstborn of all the children of Israel, have I taken them unto me.