Numbers 3:12

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

10 And thou shalt appoint Aaron and his sons, and they shall keep their priesthood: and the stranger that cometh nigh shall be put to death.

11 And Jehovah spake unto Moses, saying,

12 And I, behold, I have taken the Levites from among the children of Israel instead of all the first-born that openeth the womb among the children of Israel; and the Levites shall be mine:

13 for all the first-born are mine; on the day that I smote all the first-born in the land of Egypt I hallowed unto me all the first-born in Israel, both man and beast; mine they shall be: I am Jehovah.

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

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