Numbers 35:26-30

But if the slayer shall at any time come without the border of the city of his refuge, whither he was fled;

And the revenger of blood find him without the borders of the city of his refuge, and the revenger of blood kill the slayer; he shall not be guilty of blood:

Because he should have remained in the city of his refuge until the death of the high priest: but after the death of the high priest the slayer shall return into the land of his possession.

So these things shall be for a statute of judgment unto you throughout your generations in all your dwellings.

Whoso killeth any person, the murderer shall be put to death by the mouth of witnesses: but one witness shall not testify against any person to cause him to die.