Deuteronomy 19:6-8

New International Version

⁶ Otherwise, the avenger of blood^(A) 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. ⁷ This is why I command you to set aside for yourselves three cities.

⁸ If the Lord your God enlarges your territory,^(B) as he promised^(C) on oath to your ancestors, and gives you the whole land he promised them,