Numbers 3:6 (KJV)

These also are the generations of Aaron and Moses in the day that the LORD spake with Moses in mount Sinai.

And 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 priests which were anointed, whom he consecrated to minister in the priest's office.

And Nadab and Abihu died before the LORD, when they offered strange fire before the LORD, in the wilderness of Sinai, and they had no children: and Eleazar and Ithamar ministered in the priest's office in the sight of Aaron their father.

And the LORD spake unto Moses, saying,

And they shall keep his charge, and the charge of the whole congregation before the tabernacle of the congregation, to do the service of the tabernacle.

And they shall keep all the instruments of the tabernacle of the congregation, and the charge of the children of Israel, to do the service of the tabernacle.

And thou shalt give the Levites unto Aaron and to his sons: they are wholly given unto him out of the children of Israel.

And thou shalt appoint Aaron and his sons, and they shall wait on their priest's office: and the stranger that cometh nigh shall be put to death.

And the LORD spake unto Moses, saying,