Numbers 3:7 (KJV)

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,

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

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,

And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine;