Numbers 8:13-19

13And thou shalt set the Levites before Aaron, and before his sons, and offer them as a wave-offering to Jehovah.

14And thou shalt separate the Levites from among the children of Israel, that the Levites may be mine.

15And afterwards shall the Levites come in to do the service of the tent of meeting. And thou shalt cleanse them, and offer them as a wave-offering.

16For they are wholly given unto me from among the children of Israel; instead of every one that breaketh open the womb, instead of every firstborn among the children of Israel, have I taken them unto me.

17For all the firstborn among the children of Israel are mine, both of man and beast; on the day that I smote every firstborn in the land of Egypt, I hallowed them to myself.

18And I have taken the Levites instead of all the firstborn among the children of Israel.

19And I have given the Levites as a gift to Aaron and to his sons, from among the children of Israel, to perform the service of the children of Israel in the tent of meeting, and to make atonement for the children of Israel; that there be no plague among the children of Israel, when the children of Israel draw near to the sanctuary.