Deuteronomy 19:4-5

4Now this is the law pertaining to one who flees there in order to live, if he has accidentally killed another without hating him at the time of the accident.

5Suppose he goes with someone else to the forest to cut wood and when he raises the ax to cut the tree, the ax head flies loose from the handle and strikes his fellow worker so hard that he dies. The person responsible may then flee to one of these cities to save himself.