Numbers 8:13-18

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

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 firstborn of all the children of Israel, have I taken them unto me.

17For all the firstborn among the children of Israel are mine, both man and beast: on the day that I smote all the firstborn in the land of Egypt I sanctified them for myself.

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