Numbers 8:14-16

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