Numbers 8:14-17

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 perform the service of the tabernacle of the congregation: and thou shalt cleanse them, and offer them for an offering. 16For they are wholly given to 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 to me. 17For all the firstborn of the children of Israel are mine, both man and beast: on the day that I smote every firstborn in the land of Egypt I sanctified them for myself.