Numbers 35:30

30 If anyone kills another, the murderer shall be put to death on the evidence of witnesses; but no one shall be put to death on the testimony of a single witness.

Numbers 35:30 In-Context

28 For the slayer must remain in the city of refuge until the death of the high priest; but after the death of the high priest the slayer may return home.
29 These things shall be a statute and ordinance for you throughout your generations wherever you live.
30 If anyone kills another, the murderer shall be put to death on the evidence of witnesses; but no one shall be put to death on the testimony of a single witness.
31 Moreover you shall accept no ransom for the life of a murderer who is subject to the death penalty; a murderer must be put to death.
32 Nor shall you accept ransom for one who has fled to a city of refuge, enabling the fugitive to return to live in the land before the death of the high priest.
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.