Numbers 35:22-25

22But if anyone pushes a person suddenly, without hostility, or throws an object at him unintentionally, 23or without looking drops a heavy stone that kills him, but he was not an enemy and did not intend to harm him, 24then the congregation must judge between the slayer and the avenger of blood according to these ordinances. 25The assembly is to protect the manslayer from the hand of the avenger of blood. Then the assembly will return him to the city of refuge to which he fled, and he must live there until the death of the high priest, who was anointed with the holy oil.