Numbers 8:14-17

14In this way, you will set the Levites apart from the rest of the people of Israel, and the Levites will belong to me.

15After this, they may go into the Tabernacle to do their work, because you have purified them and presented them as a special offering.

16“Of all the people of Israel, the Levites are reserved for me. I have claimed them for myself in place of all the firstborn sons of the Israelites; I have taken the Levites as their substitutes.

17For all the firstborn males among the people of Israel are mine, both of people and of animals. I set them apart for myself on the day I struck down all the firstborn sons of the Egyptians.