Numbers 3:11-13

11The Lord said to Moses,

12“Out of all the Israelites, I have taken the Levites to be substitutes for every firstborn male offspring among them. The Levites are mine,

13because every firstborn is mine. The day I killed every firstborn male in Egypt, I set apart as holy every firstborn in Israel, whether human or animal. They will be mine. I am the Lord.”