Numbers 3:12

9 You shall give the Levites to Aaron and to his sons: they are wholly given to him on the behalf of the children of Israel.

10 You shall appoint Aaron and his sons, and they shall keep their priesthood: and the stranger who comes near shall be put to death.

11 Yahweh spoke to Moses, saying,

12 I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn who opens the womb among the children of Israel; and the Levites shall be mine:

13 for all the firstborn are mine; on the day that I struck all the firstborn in the land of Egypt I made holy to me all the firstborn in Israel, both man and animal; mine they shall be: I am Yahweh.

14 Yahweh spoke to 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 shall you number them.