Deuteronomy 19:8-10

8“And if the Lord your God enlarges your territory, as he swore to your ancestors, and gives you all the land he promised them,

9you must designate three additional cities of refuge. (He will give you this land if you are careful to obey all the commands I have given you—if you always love the Lord your God and walk in his ways.)

10That way you will prevent the death of innocent people in the land the Lord your God is giving you as your special possession. You will not be held responsible for the death of innocent people.