Numbers 3:13

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.

16 Moses numbered them according to the word of Yahweh, as he was commanded.