Numbers 8:17-19

17Every firstborn in Israel, whether human or animal, is mine. The day I killed every firstborn male in Egypt, I set them apart as holy to me.

18So I have taken the Levites as substitutes for all the firstborn sons of the Israelites.

19The Levites will be the only Israelites I give to Aaron and his sons. They will work for the Israelites at the tent of meeting. They will make peace with the Lord for the Israelites. Then no plague will strike the Israelites when they come near the holy place.”