Numbers 3:8-18

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.
14 The LORD also spoke unto Moses in the wilderness of Sinai, saying,
15 Number the sons of Levi by the houses of their fathers, by their families; thou shalt number every male from a month old and upward.
16 And Moses numbered them according to the word of the LORD, as he was commanded.
17 And these were the sons of Levi by their names: Gershon and Kohath and Merari.
18 And these are the names of the sons of Gershon by their families: Libni and Shimei.