Numbers 8:14-17

14In this way you are to set the Levites apart from the other Israelites, and the Levites will be mine.

15“After you have purified the Levites and presented them as a wave offering, they are to come to do their work at the tent of meeting.

16They are the Israelites who are to be given wholly to me. I have taken them as my own in place of the firstborn, the first male offspring from every Israelite woman.

17Every firstborn male in Israel, whether human or animal, is mine. When I struck down all the firstborn in Egypt, I set them apart for myself.