Deuteronomy 19:4-5

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.