SB 4.16.13
nadandyam dandayaty esa
sutam atma-dvisam api
dandayaty atmajam api
dandyam dharma-pathe sthitah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Since this King will always remain on the path of piety, he will be neutral to both his son and the son of his enemy. If the son of his enemy is not punishable, he will not punish him, but if his own son is punishable, he will immediately punish him.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
These are the characteristics of an impartial ruler. It is the duty of a ruler to punish the criminal and give protection to the innocent. King Prthu was so neutral that if his own son were punishable, he would not hesitate to punish him. On the other hand, if the son of his enemy were innocent, he would not engage in some intrigue in order to punish him.