Numbers 3:10-13

So you shall appoint Aaron and his sons that they may keep their priesthood, but the layman who comes near shall be put to death.” Again the LORD spoke to Moses, saying, “Now, behold, I have taken the Levites from among the sons of Israel instead of every firstborn, the firstborn 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 fatally struck all the firstborn in the land of Egypt, I sanctified to Myself all the firstborn in Israel, from the human firstborn to animals. They shall be Mine; I am the LORD.”