To Cry For Imam Hussain (as)
In Iraq, there is a small town near the place where the Tigris and Euphrates rivers meet called al-Musayyab.
A Shia man used to pass by this town from time to time whenever he went to perform ziyara of Imam Ali (as), and a Sunni man, too, was residing in this same town.
The Sunni man quite often used to make fun of the Shia man whenever he saw him going to visit the shrine of Imam Ali (as), so much so that he dared once to speak ill of the holy Imam (as).
So the Shia man was very angry and he complained about this matter to Imam Ali (as) during one of his visits.
During that night, he saw the Imam (as) in a vision and once more complained to him about this matter. The Imam (as) said, "He [the Sunni man] has don us a favor, and we cannot punish him in the life of this world no matter what sins he commits."
The Shia man asked, "What favor is it? Did he do you a favor when he dared to speak ill of you?"
The Imam (as) said, "No, but he was sitting one day at the meeting place of both rivers, the Tigris and the Euphrates, and he was looking at the Euphrates. He remembered the Karbala story and the thirst of Imam Hussain (as), so he said this to himself: "Omer ibn Sa'd was wrong when he killed those men as they were thirsty, it would have been better for him to give them water and then kill them." And then a tear trickled down his eye out of grief for Imam Hussain (as); therefore, it became mandatory on us never to punish him in this life."
The Shia man says, "I woke up from my sleep and returned to al-Musayyab and met the Sunni man on the road who said this to me in ridicule: "Did you visit your Imam, and did you convey our message to him?" I said, "Yes, I conveyed your message to him, and I carry his message to you."
The Sunni man laughed and said, "What is this message which you carry to me?"
The Shia man narrated to him the incident from beginning to end.
The Sunni man lowered his head to the ground and kept thinking: "O Lord! Nobody at that moment was near me, and I did not talk about this incident to anyone, so how did the Imam (as) become familiar with it?"
Then he said, "I testify that there is no god save Allah, and that Muhammad is the messenger of Allah, and that Ali, the Commander of the Faithful, is a friend of Allah and the wasi of the Messenger of Allah."
Source - The Model Of The Gnostics (Biography of Ayatollah Bahjat)