Deuteronomy 19:9-11

9you must continue to obey all the commands that I am giving you today. You must continue to love the Lord your God and live in a way that pleases him. Then, when your land becomes bigger, you must choose three more cities to be safe cities.

10You must not punish with death people who are not guilty. That would make you guilty yourselves. That must not happen in the land that the Lord your God is giving you.

11But perhaps somebody does hate another person. He may hide himself so that he can catch his enemy. Then he may attack him and kill him. After that, he may run to one of the safe cities.