Numbers 3:10-13

ASV(i) 10 And thou shalt appoint Aaron and his sons, and they shall keep their priesthood: and the stranger that cometh nigh shall be put to death.
11 And Jehovah spake unto Moses, saying, 12 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: 13 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.