"Then you shall judge yourself," the king answered. "That is the most difficult thing of all. It is much more… — Antoine de Saint-Exupéry

"Then you shall judge yourself," the king answered. "That is the most difficult thing of all. It is much more difficult to judge oneself than to judge others. If you succeed in judging yourself rightly, then you are indeed a man of true wisdom.

In the original (French)

Tu te jugeras donc toi-même, lui répondit le roi. C'est le plus difficile. Il est bien plus difficile de se juger soi-même que de juger autrui. Si tu réussis à bien te juger, c'est que tu es un véritable sage.

Source: Wikiquote · CC BY-SA 4.0