Deuteronomy 19:4-7

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. Otherwise, the dead person’s relative who has the duty of punishing a murderer might be angry and chase him. If the city is far away, the relative might catch and kill the person, even though he should not be killed because there was no intent to kill his neighbor. This is why I command you to choose these three cities.