Deuteronomy 19:4-13

4 If you accidentally kill someone who is not your enemy, you may escape to any of these cities and be safe.
5 For example, if two of you go into the forest together to cut wood and if, as one of you is chopping down a tree, the ax head comes off the handle and kills the other, you can run to one of those three cities and be safe.
6 If there were only one city, the distance to it might be too great, and the relative who is responsible for taking revenge for the killing might catch you and angrily kill an innocent person. After all, it was by accident that you killed someone who was not your enemy.
7 This is why I order you to set aside three cities.
8 "When the Lord your God enlarges your territory, as he told your ancestors he would, and gives you all the land he has promised,
9 then you are to select three more cities. (He will give you this land if you do everything that I command you today and if you love the Lord your God and live according to his teachings.)
10 Do this, so that innocent people will not die and so that you will not be guilty of putting them to death in the land that the Lord is giving you.
11 "But suppose you deliberately murder your enemy in cold blood and then escape to one of those cities for protection.
12 In that case, the leaders of your own town are to send for you and hand you over to the relative responsible for taking revenge for the murder, so that you may be put to death.
13 No mercy will be shown to you. Israel must rid itself of murderers, so that all will go well.
Scripture taken from the Good News Translation - Second Edition, Copyright 1992 by American Bible Society. Used by Permission.