Numbers 3:12-14 New Revised Standard Version (NRSV)
12 I hereby accept the Levites from among the Israelites as substitutes for all the firstborn that open the womb among the Israelites. The Levites shall be mine, 13 for all the firstborn are mine; when I killed all the firstborn in the land of Egypt, I consecrated for my own all the firstborn in Israel, both human and animal; they shall be mine. I am the Lord.
A Census of the Levites
14 Then the Lord spoke to Moses in the wilderness of Sinai, saying: