3:1 These are the generations of Aaron and Moses at the time when the Lord spoke with Moses on Mount Sinai. 2 These are the names of the sons of Aaron: Nadab the firstborn, and Abihu, Eleazar, and Ithamar. 3 These are the names of the sons of Aaron, the anointed priests, whom he ordained to serve as priests. 4 But Nadab and Abihu died before the Lord when they offered unauthorized fire before the Lord in the wilderness of Sinai, and they had no children. So Eleazar and Ithamar served as priests in the lifetime of Aaron their father.
5 And the 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 They shall keep guard over him and over the whole congregation before the tent of meeting, as they minister at the tabernacle. 8 They shall guard all the furnishings of the tent of meeting, and keep guard over the people of Israel as they minister at the tabernacle. 9 And you shall give the Levites to Aaron and his sons; they are wholly given to him from among the people of Israel. 10 And you shall appoint Aaron and his sons, and they shall guard their priesthood. But if any outsider comes near, he shall be put to death.”
11 And the Lord spoke to Moses, saying, 12 “Behold, I have taken the Levites from among the people of Israel instead of every firstborn who opens the womb among the people of Israel. The Levites shall be mine, 13 for all the firstborn are mine. On the day that I struck down all the firstborn in the land of Egypt, I consecrated for my own all the firstborn in Israel, both of man and of beast. They shall be mine: I am the Lord.”
14 And the Lord spoke to Moses in the wilderness of Sinai, saying, 15 “List the sons of Levi, by fathers' houses and by clans; every male from a month old and upward you shall list.” 16 So Moses listed 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 clans: Libni and Shimei. 19 And the sons of Kohath by their clans: Amram, Izhar, Hebron, and Uzziel. 20 And the sons of Merari by their clans: Mahli and Mushi. These are the clans of the Levites, by their fathers' houses. (ESV)