Imam Hasan (‘a) and Imam Husain (‘a) were still children, when, one day, while on their way to the mosque, to offer the congregational prayers, they spied an aged person performing ablution ( wuzu) for the prayer. It struck them both that he was not performing it correctly. They stopped, realizing it was their duty to correct him.
They also realized that if they told him that his wuzu was not correct it could make him feel humiliated, and he might either refuse to admit it and stubbornly persist in doing it his way, i.e. the wrong way; or feel unhappy whenever he performed wuzu correctly, by recalling the humiliating moment when he was corrected by two children. They knew that criticism causes resistance and stubbornness, and seldom mends, thus fails to achieve the desired result. On the contrary, courtesy and humility can overpower the most ignorant, most resistant and most arrogant.
Imam Hasan (‘a) whispered something in his brother’s ear and they both took a tumbler full of water and stood within hearing distance of the old man. ‘I can perform ablution better than you,’ said Hasan (‘a). ‘No. I can perform it in the best way possible,’ said Husain (‘a). ‘Let us find somebody to judge who is better,’ said Hasan (‘a).
They walked towards the old man and greeted him, ‘Assalam o alaikum, worthy Muslim! Can you do us a favor? My brother and I want you to judge which of us performs ablution as ordained? Will you watch us very carefully and point out any discrepancy that you notice?’ ‘Certainly,’ replied the old man, overjoyed to judge the Holy Prophet’s (s) grandchildren.
First Imam Husain (‘a) performed ablution very slowly, intentionally. The old man, immediately, realized how wrong he was, and decided to correct his own mistakes after observing the elder child and being sure. Then Imam Hasan (‘a) performed ablution in exactly the same way with the same speed.
The old man tearfully embraced them and said, ‘Noble children of the noblest family! Who could have pointed out my mistakes to me more affectionately and convincingly? May Allah bless you and reward you for guiding me aright.’
Reference : Anecdotes of The Ahlul Bayt (Daastaan e Raastaan ) by Ayatullah Murtadha Mutahhari