Deuteronomy 19:6-8

New Catholic Bible

⁶ Otherwise, the relative who seeks blood vengeance might seek him out while he is still blind with rage, and if the distance be too great he would then be able to catch up with him and kill him even though he did not really deserve to die since he did not bear the man any previous ill will.^{[a] 7} This is why I command you to set aside three cities for yourselves.

⁸ If the Lord, your God, adds to your territory, as he promised to do to your ancestors, and he gives you all the land that he promised to give to your fathers,