Numbers 3:10

10 But you shall make a register of Aaron and his descendants; it is they who shall attend to the priesthood, and any outsider who comes near shall be put to death.

Numbers 3:10 In-Context

8 they shall be in charge of all the furnishings of the tent of meeting, and attend to the duties for the Israelites as they do service at the tabernacle.
9 You shall give the Levites to Aaron and his descendants; they are unreservedly given to him from among the Israelites.
10 But you shall make a register of Aaron and his descendants; it is they who shall attend to the priesthood, and any outsider who comes near shall be put to death.
11 Then the Lord spoke to Moses, saying:
12 I hereby accept the Levites from among the Israelites as substitutes for all the firstborn that open the womb among the Israelites. The Levites shall be mine,
New Revised Standard Version Bible, copyright 1989, Division of Christian Education of the National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved.