Numbers 3:11 (KJV)

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.

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 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;

Because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast: mine shall they be: I am the LORD.

And the LORD spake unto Moses in the wilderness of Sinai, saying,

Number the children of Levi after the house of their fathers, by their families: every male from a month old and upward shalt thou number them.

And Moses numbered them according to the word of the LORD, as he was commanded.