Numbers 3:12

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

10 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.

11 And the LORD spake unto Moses, saying,

12 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;

13 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.

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

15 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.