Numbers 35:22-27

But if he thrust him suddenly without enmity, or hurled upon him anything without lying in wait, or with any stone, whereby a man may die, seeing him not, and cast it upon him, so that he died, and he was not his enemy, neither sought his harm; then the congregation shall judge between the smiter and the avenger of blood according to these ordinances; and the congregation shall deliver the manslayer out of the hand of the avenger of blood, and the congregation shall restore him to his city of refuge, whither he was fled: and he shall dwell therein until the death of the high priest, who was anointed with the holy oil. But if the manslayer shall at any time go beyond the border of his city of refuge, whither he fleeth, and the avenger of blood find him without the border of his city of refuge, and the avenger of blood slay the manslayer; he shall not be guilty of blood