Numbers 3:11-13

11And the Lord said to Moses,

12“Look, I have chosen the Levites from among the Israelites to serve as substitutes for all the firstborn sons of the people of Israel. The Levites belong to me,

13for all the firstborn males are mine. On the day I struck down all the firstborn sons of the Egyptians, I set apart for myself all the firstborn in Israel, both of people and of animals. They are mine; I am the Lord.”