Deuteronomy 19:8-10
8 If the Lord your God enlarges your territory, as he promised on oath to your forefathers, and gives you the whole land he promised them,
9 because you carefully follow all these laws I command you today—to love the Lord your God and to walk always in his ways—then you are to set aside three more cities.
10 Do this so that innocent blood will not be shed in your land, which the Lord your God is giving you as your inheritance, and so that you will not be guilty of bloodshed.