Deuteronomy 19:4-7

This is the guideline for the murderer who flees there to take refuge: He has to have killed his neighbor without premeditation and with no history of bad blood between them. For instance, a man goes with his neighbor into the woods to cut a tree; he swings the ax, the head slips off the handle and hits his neighbor, killing him. He may then flee to one of these cities and save his life. If the city is too far away, the avenger of blood racing in hot-blooded pursuit might catch him since it’s such a long distance, and kill him even though he didn’t deserve it. It wasn’t his fault. There was no history of hatred between them. Therefore I command you: Set aside the three cities for yourselves.