Numbers 3:11-13

11The LORD also said to Moses, 12“I have taken the Levites from among the Israelites in place of the first male offspring of every Israelite woman. The Levites are mine,

13for all the firstborn are mine. When I struck down all the firstborn in Egypt, I set apart for myself every firstborn in Israel, whether human or animal. They are to be mine. I am the LORD .”