Numbers 8:13-19

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

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 do the service of the tent of meeting: and thou shalt cleanse them, and offer them for a wave-offering. 16For 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. 17For all the first-born among the children of Israel are mine, both man and beast: on the day that I smote all the first-born in the land of Egypt I sanctified them for myself. 18And I have taken the Levites instead of all the first-born 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 do 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 come nigh unto the sanctuary.