Numbers 3:10-13

You are to appoint Aaron and his sons to carry out their priestly responsibilities, but any unauthorized person who comes near the sanctuary is to be put to death.”  The LORD spoke to Moses: “See, I have taken the Levites from the Israelites in place of every firstborn Israelite from the womb.  The Levites belong to me, because every firstborn belongs to me. At the time I struck down every firstborn in the land of Egypt,  I consecrated every firstborn in Israel to myself, both man and animal. They are mine; I am the LORD.”