Numbers 3:9

5And the Lord spake unto Moses, saying,

6Bring the tribe of Levi near, and present them before Aaron the priest, that they may minister unto him.

7And 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.

8And 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.

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

10And 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.

11And the Lord spake unto Moses, saying,

12And 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;

13Because 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.