Numbers 3:13

10 And give orders that Aaron and his sons are to keep their place as priests; any strange person who comes near is to be put to death.

11 And the Lord said to Moses,

12 See, I have taken the Levites out of the children of Israel to be mine in place of the first sons of the children of Israel;

13 For all the first sons are mine; on the day when I put to death all the first sons in the land of Egypt, I took for myself every first male birth of man and beast. They are mine; I am the Lord.

14 And the Lord said to Moses in the waste land of Sinai,

15 Let all the children of Levi be numbered by their families and their fathers' houses; let every male of a month old and over be numbered.

16 So Moses did as the Lord said, numbering them as he had been ordered.