Deuteronomy 19:9-11

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.

11"But if anyone hates his neighbor and lies in wait for him and attacks him and strikes him fatally so that he dies, and he flees into one of these cities,