The Sons of Aaron
This is the lineage of Aaron and Moses at the time when the LORD spoke with Moses on Mount Sinai. These are the names of the sons of Aaron: Nadab the firstborn, and Abihu, Eleazar, and Ithamar; these are the names of the sons of Aaron, the anointed priests, whom he ordained to minister as priests. Nadab and Abihu died before the LORD when they offered unholy fire before the LORD in the wilderness of Sinai, and they had no children. Eleazar and Ithamar served as priests in the lifetime of their father Aaron.
The Duties of the Levites
Then the LORD spoke to Moses, saying: Bring the tribe of Levi near, and set them before Aaron the priest, so that they may assist him. They shall perform duties for him and for the whole congregation in front of the tent of meeting, doing service at the tabernacle; they shall be in charge of all the furnishings of the tent of meeting, and attend to the duties for the Israelites as they do service at the tabernacle. You shall give the Levites to Aaron and his descendants; they are unreservedly given to him from among the Israelites. But you shall make a register of Aaron and his descendants; it is they who shall attend to the priesthood, and any outsider who comes near shall be put to death.
Then the LORD spoke to Moses, saying: I hereby accept the Levites from among the Israelites as substitutes for all the firstborn that open the womb among the Israelites. The Levites shall be mine, for all the firstborn are mine; when I killed all the firstborn in the land of Egypt, I consecrated for my own all the firstborn in Israel, both human and animal; they shall be mine. I am the LORD.