Numbers 35:30-33

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

31You are not to accept a ransom for the life of someone who is guilty of murder; he must be put to death.

32Neither should you accept a ransom for the person who flees to his city of refuge, allowing him to return and live in the land before the death of the high priest.

33“Do not defile the land where you live, for bloodshed defiles the land, and there can be no atonement for the land because of the blood that is shed on it, except by the blood of the person who shed it.