From: Anecdotes of the Pious Ones, # 75, v.1
The poor woman was breathless as she was carrying the bagfull of water to her house. A stranger took the bag and carried it for her.
Her young children were awaiting mother's arrival. The door opened. They saw that a stranger, carrying water, entered their house along with the mother.
The stranger put the water down and asked her: "It seems that you don't have a husband to bring water. What happened?"
"My husband was a soldier. 'Imam Ali ibn Abi Talib (as) sent him to one of the borders and he was killed there. Now, here I am with these young children."
The stranger did not say anything else. He bent his head, said farewell and left. But he didn't forget, even for a moment, that woman and those children. He couldn't sleep that night. The next day, he carried a bag, full of meat, flour, and dates and went to her house. He knocked.
"Who is it?"
"That servant of God who carried water yesterday. I have brought some food for the children."
"May Allah be pleased with you. May He judge between me and 'Ali."
The door opened and the stranger entered and said: "I wish to do some good deed. If you permit, I shall kneel and bake the bread, or babysit."
"Well. I am better at baking. Why don't watch the kids till I finish."
The woman left to prepare the dough. The stranger cooked the meat he had brought and fed it and the dates to the children.
As he was putting a bite in their mouths, he was saying: "My child, forgive 'Ali ibn-e Abi Talib who has been neglectful of your rights."
When the woman had mixed the flour, she called: "O' servant of Allah. Put the fire in the oven."
The stranger put fire in the oven, and brought his face close to the flames: "Feel this fire. This is the punishment of one who neglects orphans and widows."
At this time, a woman from the neighbourhood walked in and recognized the stranger. She told the woman: "Woo unto you. Do you know that it is Imam Ameer al Mu'mineen 'Ali ibn-e Abi Talib (as) who is helping you?"
The woman approached him: "Shame unto me. Please forgive me."
"I should ask for forgiveness from you. I have neglected your right." 
 Bihar al Anwar. v. 7, section 103, p. 597