Deuteronomy 19:8-10

8And if the Lord your God enlarges your territory, as he has sworn to your fathers, and gives you all the land that he promised to give to your fathers—

9provided you are careful to keep all this commandment, which I command you today, by loving the Lord your God and by walking ever in his ways—then you shall add three other cities to these three,

10lest innocent blood be shed in your land that the Lord your God is giving you for an inheritance, and so the guilt of bloodshed be upon you.