Deuteronomy 19:6-8

New International Version - UK

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 neighbour 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, as he promised on oath to your ancestors, and gives you the whole land he promised them,