Numbers 3:5

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.

Bring the tribe of Levi near, and present them before Aaron the priest, that they may minister unto him.

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.