Numbers 3:6-13

⁶“Bring the tribe of Levi forward and present them before Aaron the priest, that they may serve him. ⁷They shall perform the duties for him and for the whole congregation in front of the tent of meeting, to do the service of the tabernacle. ⁸They shall also take care of all the furnishings of the tent of meeting, along with the duties of the sons of Israel, to do the service of the tabernacle. ⁹So you shall assign the Levites to Aaron and to his sons; they are exclusively assigned to him from the sons of Israel. ¹⁰So you shall appoint Aaron and his sons that they may keep their priesthood, but the layman who comes near shall be put to death.”

¹¹Again the Lord spoke to Moses, saying, ¹²“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. ¹³For 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.”