Likutei Moharan 19:2/ליקוטי מוהר"ן י״ט:ב׳

12. Now, the tzaddik has a pure countenance.> Everyone can see his own face in the face of <the tzaddik>, as [he could] in a mirror. As a result, even without rebuke and without reproof, his friend will feel remorse for his deeds just by having looked into <the tzaddik’s> face. This is because by looking into his face, a person will see himself as if in a mirror and, [realizing] how he is immersed in darkness, <will feel remorse>.