Numbers 8:14

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

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

And 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 [for] an offering.

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