It is related that a black man confessed before Amir ul-Mu'minin (a) that he had committed larceny. Imam Ali (a) tried to provide excuses that may exempt him from the punishment of larceny. But, the black man insisted and confessed completely; hence, A mir ul-Mu'minin (a) had to subject him to the doctrinal provision, which was cutting of the hand.
In his way back, that black man was murmuring, “My hand has been cut by the commander of the faithful believers, the leader of the pious, the chief of the white-forheaded honorables, the master of the religion, the head of the prophets’ successors…” As a l-Hasan and al-Hussein heard these words, they conveyed them to their father, who summoned that black man to ask him about that praise. The black man said:
“Amir ul-Mu'minin, you have purified me. Your love has been mixed with my flesh and blood so composedly that it cannot depart my heart even if you cut me into pieces.” … Safinat ul-Bihar; part 2 page 716 ) 

Reference :The Ahl Ul-Bayt Ethical Role-Models