Numbers 3:5-13 - The Assignment Of The Levites
5 And the LORD spoke unto Moses, saying, 6 Bring the tribe of Levi near and present them before Aaron, the priest, that they may minister unto him. 7 And they shall keep his commandment and the commandment of the whole congregation before the tabernacle of the testimony, to minister in the service of the tabernacle. 8 And they shall keep all the instruments of the tabernacle of the testimony and the charge of the sons of Israel, to minister in the service of the tabernacle. 9 And thou shalt give the Levites unto Aaron and to his sons; they are completely given unto him out of the sons of Israel. 10 And thou shalt appoint Aaron and his sons that they shall guard their priesthood; and the stranger that comes near shall be put to death.
11 And the LORD spoke unto Moses, saying, 12 Behold, I have taken the Levites from among the sons of Israel instead of all the firstborn that open the womb among the sons of Israel; therefore the Levites shall be mine; 13 because all the firstborn are mine; from the day that I smote all the firstborn in the land of Egypt, I sanctified unto me all the firstborn in Israel, both of men and of animals; they shall be mine: I am the LORD.