Numbers 8:14-17

14That is how you will make the Levites separate from the other Israelites. And then the Levites will be mine.

15Then after that the Levites can go in to work in the Tent of Meeting. But you must make them clean and you must bring them to me as a special gift.

16That is because all the Levites among the Israelites are mine. All the Israelites' firstborn sons are mine. But I have taken the Levites for myself instead.

17Every firstborn son among the Israelites is mine. And all the firstborn male animals are mine too. On the day that I killed all the firstborn sons in Egypt I made them special for myself.