Numbers 3:12-13

And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the womb among the children of Israel; and the Levites shall be mine: for all the firstborn are 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 man and beast: mine they shall be; I am the LORD.