Numbers 3:11-13

11And Jehovah spoke to Moses, saying,

12And I, behold, I have taken the Levites from among the children of Israel instead of every firstborn that breaketh open the womb among the children of Israel, and the Levites shall be mine;

13for every firstborn is mine. On the day that I smote all the firstborn in the land of Egypt, I hallowed unto me all the firstborn in Israel, both of man and beast; mine shall they be: I am Jehovah.