Deuteronomy 19:4-7

4“If someone kills another person unintentionally, without previous hostility, the slayer may flee to any of these cities to live in safety.

5For example, suppose someone goes into the forest with a neighbor to cut wood. And suppose one of them swings an ax to chop down a tree, and the ax head flies off the handle, killing the other person. In such cases, the slayer may flee to one of the cities of refuge to live in safety.

6“If the distance to the nearest city of refuge is too far, an enraged avenger might be able to chase down and kill the person who caused the death. Then the slayer would die unfairly, since he had never shown hostility toward the person who died.

7That is why I am commanding you to set aside three cities of refuge.