Numbers 3:12-13

“Now, behold, I have taken the Levites from among the sons of Israel instead of every firstborn, the first issue of the womb among the sons of Israel. So the Levites shall be Mine. For all the firstborn are Mine; on the day that I struck down all the firstborn in the land of Egypt, I sanctified to Myself all the firstborn in Israel, from man to beast. They shall be Mine; I am the LORD.”