Numbers 3:14

12“Behold, I have taken the Levites from among the children of Israel instead of all the firstborn who open the womb among the children of Israel; and the Levites shall be mine,

13for all the firstborn are mine. On the day that I struck down all the firstborn in the land of Egypt I made holy to me all the firstborn in Israel, both man and animal. They shall be mine. I am the LORD.”

14The LORD spoke to Moses in the wilderness of Sinai, saying,

15“Count the children of Levi by their fathers’ houses, by their families. You shall count every male from a month old and upward.”

16Moses counted them according to the LORD’s word, as he was commanded.