Numbers 3:12-16

And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine;

Because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast: mine shall they be: I am the LORD.

And the LORD spake unto Moses in the wilderness of Sinai, saying,

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

And Moses numbered them according to the word of the LORD, as he was commanded.