Numbers 8:13-18

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.