Numbers 3:15 (KJV)

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;

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,

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

And these were the sons of Levi by their names; Gershon, and Kohath, and Merari.

And these are the names of the sons of Gershon by their families; Libni, and Shimei.

And the sons of Kohath by their families; Amram, and Izehar, Hebron, and Uzziel.

And the sons of Merari by their families; Mahli, and Mushi. These are the families of the Levites according to the house of their fathers.