Numbers 35:29-31

Young's Literal Translation

²⁹ `And these things have been to you for a statute of judgment to your generations, in all your dwellings:

³⁰ whoso smiteth a person, by the mouth of witnesses doth [one] slay the murderer; and one witness doth not testify against a person -- to die.

³¹ `And ye take no atonement for the life of a murderer who [is] condemned -- to die, for he is certainly put to death;