Numbers 35:31 (KJV)

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.

And ye shall take no satisfaction for him that is fled to the city of his refuge, that he should come again to dwell in the land, until the death of the priest.

So ye shall not pollute the land wherein ye are: for blood it defileth the land: and the land cannot be cleansed of the blood that is shed therein, but by the blood of him that shed it.

Defile not therefore the land which ye shall inhabit, wherein I dwell: for I the LORD dwell among the children of Israel.