Numbers 3:12-15

And I, behold, I have taken the Levites from among the children of Israel instead of all the first-born that openeth the womb among the children of Israel; and the Levites shall be mine: for all the first-born are mine; on the day that I smote all the first-born in the land of Egypt I hallowed unto me all the first-born in Israel, both man and beast; mine they shall be: I am Jehovah. And Jehovah spake unto Moses in the wilderness of Sinai, saying, Number the children of Levi by their fathers’ houses, by their families: every male from a month old and upward shalt thou number them.