Numbers 8:14-17

14And so you are to separate the Levites from among the Israelites, and the Levites will be mine.

15“After this, the Levites will go in to do the work of the tent of meeting. So you must cleanse them and offer them like a wave offering.

16For they are entirely given to me from among the Israelites. I have taken them for myself instead of all who open the womb, the firstborn sons of all the Israelites.

17For all the firstborn males among the Israelites are mine, both humans and animals; when I destroyed all the firstborn in the land of Egypt I set them apart for myself.