This is the line of Aaron and Moses at the time that the Lord spoke with Moses on Mount Sinai. These were the names of Aaron's sons: Nadab, the first-born, and Abihu, Eleazar and Ithamar; those were the names of Aaron's sons, the anointed priests who were ordained for priesthood. But Nadab and Abihu died by the will of the Lord, when they offered alien fire before the Lord in the wilderness of Sinai; and they left no sons. So it was Eleazar and Ithamar who served as priests in the lifetime of their father Aaron.
The Lord spoke to Moses, saying: Advance the tribe of Levi and place them in attendance upon Aaron the priest to serve him. They shall perform duties for him and for the whole community before the Tent of Meeting, doing the work of the Tabernacle. They shall take charge of all the furnishings of the Tent of Meeting--a duty on behalf of the Israelites--doing the work of the Tabernacle. You shall assign the Levites to Aaron and to his sons: they are formally assigned to him from among the Israelites. You shall make Aaron and his sons responsible for observing their priestly duties; and any outsider who encroaches shall be put to death.
The Lord spoke to Moses, saying: I hereby take the Levites from among the Israelites in place of all the first-born, the first issue of the womb among the Israelites: the Levites shall be Mine. For every first-born is Mine: at the time that I smote every first-born in the land of Egypt, I consecrated every first-born in Israel, man and beast, to Myself, to be Mine, the Lord's.