Numbers 3:9-12King James Version (KJV)
9 And thou shalt give the Levites unto Aaron and to his sons: they are wholly given unto him out of the children of Israel.
10 And thou shalt appoint Aaron and his sons, and they shall wait on their priest's office: and the stranger that cometh nigh shall be put to death.
11 And the Lord spake unto Moses, saying,
12 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;