Deuteronomy 19:9-11

9when you take care to do all this mitzvah that I am commanding you today, to love Adonai your God and to always walk in His ways. Then you are to add three more cities for yourself, besides these three.

10Then innocent blood will not be shed within your land that Adonai your God is giving you as an inheritance, and there would be blood upon you.

11“But suppose someone hates his neighbor, waits in hiding for him, rises up against him, strikes him dead, then flees to one of these cities.