Deuteronomy 19:4; Deuteronomy 19:6

4 This is the rule concerning the man who kills another and flees there to save his life--one who kills his neighbor unintentionally, without malice aforethought.
6 Otherwise, the avenger of blood might pursue him in a rage, overtake him if the distance is too great, and kill him even though he is not deserving of death, since he did it to his neighbor without malice aforethought.