Numbers 8:13-18

13“And you shall stand the Levites before Aaron and his sons, and then offer them like a wave offering to the Lord.

14Thus you shall separate the Levites from among the children of Israel, and the Levites shall be Mine.

15After that the Levites shall go in to service the tabernacle of meeting. So you shall cleanse them and offer them like a wave offering.

16For they are wholly given to Me from among the children of Israel; I have taken them for Myself instead of all who open the womb, the firstborn of all the children of Israel.

17For all the firstborn among the children of Israel are Mine, both man and beast; on the day that I struck all the firstborn in the land of Egypt I sanctified them to Myself.

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