Numbers 3:5-13 - The Assignment Of The Levites
5 And LORD spoke to Moses, saying, 6 Bring the tribe of Levi near, and set them before Aaron the priest, that they may minister to him. 7 And they shall keep his charge, and the charge of the whole congregation before the tent of meeting, to do the service of the tabernacle. 8 And they shall keep all the furniture of the tent of meeting, and the charge of the sons of Israel, to do the service of the tabernacle. 9 And thou shall give the Levites to Aaron and to his sons. They are wholly given to him on the behalf of the sons of Israel. 10 And thou shall appoint Aaron and his sons, and they shall keep their priesthood. And the stranger who comes near shall be put to death.
11 And LORD spoke to Moses, saying, 12 And I, behold, I have taken the Levites from among the sons of Israel instead of all the first-born who opens the womb among the sons of Israel. And the Levites shall be mine, 13 for all the first-born are mine. On the day that I smote all the first-born in the land of Egypt I hallowed to me all the first-born in Israel, both man and beast. They shall be mine. I am LORD.