Numbers 3:12-13

Behold, I have taken the Levites from among the sons of Israel instead of all the firstborn that open the womb among the sons of Israel; therefore the Levites shall be mine; because all the firstborn are mine; from the day that I smote all the firstborn in the land of Egypt, I sanctified unto me all the firstborn in Israel, both of men and of animals; they shall be mine: I am the LORD.