Numbers 8:14-17

14In this way you are to separate the Levites from the rest of the Israelites so that the Levites will belong to me.

15After that the Levites may come to serve at the tent of meeting, once you have ceremonially cleansed them and presented them as a presentation offering.

16For they have been exclusively assigned to me from the Israelites. I have taken them for myself in place of all who come first from the womb, every Israelite firstborn.

17For every firstborn among the Israelites is mine, both man and animal. I consecrated them to myself on the day I struck down every firstborn in the land of Egypt.