Numbers 35:30-33

30“Whoever kills any person, the murderer must be put to death by the testimony of witnesses; but one witness cannot testify against any person to cause him to be put to death.

31Moreover, you must not accept a ransom for the life of a murderer who is guilty of death; he must surely be put to death.

32And you must not accept a ransom for anyone who has fled to a town of refuge, to allow him to return home and live on his own land before the death of the high priest.

33“You must not pollute the land where you live, for blood defiles the land, and the land cannot be cleansed of the blood that is shed there, except by the blood of the person who shed it.