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 tent of meeting: and thou shalt cleanse them, and offer them for a wave-offering. For they are wholly given unto me from among the children of Israel; instead of all that openeth the womb, even the first-born of all the children of Israel, have I taken them unto me.