Numbers 3:5-13 - The Assignment Of The Levites
5 And the LORD spake unto Moses, saying, 6 "Bring the tribe of Levi, and set them before Aaron the priest, and let them serve him and wait upon him - 7 and upon all the multitude - before the tabernacle of witness, to do the service of the habitation. 8 And they shall wait upon all the apparel of the tabernacle of witness, and upon the children of Israel, to do the service of the habitation. 9 And thou shalt give the Levites unto Aaron and his sons, for they are given unto him of the children of Israel. 10 And thou shalt appoint Aaron and his sons to wait on their priests' office: and the stranger that cometh nigh shall die for it."
11 And the LORD spake unto Moses, saying, 12 "Behold, I have taken the Levites, from among the children of Israel, for all the firstborn that openeth the matrix among the children of Israel: so that the Levites shall be mine. 13 Because all the firstborn are mine - for the same day that I smote all the firstborn in the land of Egypt, I hallowed unto me all the firstborn in Israel, both man and beast, and mine they shall be: for I am the LORD."