Numbers 8:13-19

13“You are to have the Levites stand before Aaron and his sons, and you are to present them before the Lord as a presentation offering.

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.

18But I have taken the Levites in place of every firstborn among the Israelites.

19From the Israelites, I have given the Levites exclusively to Aaron and his sons to perform the work for the Israelites at the tent of meeting and to make atonement on their behalf, so that no plague will come against the Israelites when they approach the sanctuary.”