Deuteronomy 19:4-5

This is the rule for someone who kills another person and runs to one of these cities in order to save his life. But the person must have killed a neighbor without meaning to, not out of hatred. For example, suppose someone goes into the forest with a neighbor to cut wood and swings an ax to cut down a tree. If the ax head flies off the handle, hitting and killing the neighbor, the one who killed him may run to one of these cities to save his life.