Deuteronomy 19:4-5

4This is the rule concerning anyone who kills a person and flees there for safety—anyone who kills a neighbor unintentionally, without malice aforethought. 5For instance, a man may go into the forest with his neighbor to cut wood, and as he swings his ax to fell a tree, the head may fly off and hit his neighbor and kill him. That man may flee to one of these cities and save his life.