Numbers 3:12-13

12“Now behold, I Myself have taken the Levites from among the children of Israel instead of every firstborn who opens the womb among the children of Israel. Therefore the Levites shall be Mine,

13because all the firstborn are Mine. On the day that I struck all the firstborn in the land of Egypt, I sanctified to Myself all the firstborn in Israel, both man and beast. They shall be Mine: I am the Lord.”