Numbers 35:30-33

30If anyone kills a person, the murderer is to be put to death on the testimony of the witnesses. But no one is to be put to death based on the testimony of a lone witness.

31You are not to accept a ransom for the life of a murderer who deserves to die; he must surely be put to death. 32Nor should you accept a ransom for the person who flees to a city of refuge and allow him to return and live on his own land before the death of the high priest.

33Do not pollute the land where you live, for bloodshed pollutes the land, and no atonement can be made for the land on which the blood is shed, except by the blood of the one who shed it.