Evidences for Imam Ali’s Guardianship
All evidences indicate that it was Allahâ€™s will that â€˜Aliâ€™s guardianship be the trial for the Muslims, as it was the reason behind every dispute and controversy erupted. And since Allah is subtile toward His bondmen, never taking the latters to task for what the earliers did, therefore He made His wisdom apparent and encompassed that incident (of Ghadir Khumm) with other splendid miracles-like events, so as to be an incentive for the Ummah, and the contemporary people would convey them and the followers would take an example of them, hoping that they might be guided to truth through searching and investigation.
It is related to punishing whoever denying â€˜Aliâ€™s guardianship, after the prevalence of the news of Ghadir Khumm, and designation of al-â€™ImÄm â€˜Ali as a caliph over the Muslims, with the Messengerâ€™s telling the people: The attendant should inform the absent.
When this news reached al-HÄrith ibn al-Nuâ€˜mÄn al-Fahri, it displeased him.  So he rushed toward the Messenger of Allah, made his camel kneel down before the Mosque door, and entered upon the Prophet (S). After saluting him, he said: O MuÙ‡ammad! You ordered us to witness that no god is there but Allah and you are the Messenger of Allah, and we accepted this from you. You also commanded us to perform five prayers day and night, fast the Month of RamadÄn, perform pilgrimage to Allahâ€™s House, and pay the alms-due out of our money and properties and so we did. But you were not satisfied with all this, till you one day surprised us with raising the hand of your cousin; preferring him over other people, and said: "Of whomever I am a master, â€˜Ali also is his master." Is this from you or ordained by Allah?
The Messenger of Allah (S), whose eyes turned red, replied: By Allah, Who no god is there but He. It is ordained by Allah, and not from me (repeating it three times)
On standing up, al-HÄrith said: "O Allah, if what is uttered by MuÙ‡ammad being true, send against us stones from the sky or bring us a painful doom."
Then he (S) said: By Allah, before reaching his camel, Allah pelted him with a stone from the heaven, which fell upon the top of his head and went out from his posterior, causing his death. Thereat these verses were revealed by Allah the Almighty: "Demanded, a demander, the chastisement inevitable. For, the disbelievers against it, there is no repeller." (70:1-2)
This episode is reported by a large number of Ahl al-Sunnah â€˜ulamÄâ€™, other than those we referred to.  Whoever desires to go through more references, can refer to the book al-Ghadir of al-â€˜AllÄmah al-â€™Amini.
It is related to the punishment of anyone concealing the witness regarding the incident of al-Ghadir, and was inflicted by the imprecation uttered by al-â€™ImÄm â€˜Ali. That was when al-â€™ImÄm â€˜Ali, during a memorable day, rose up, and gathered people at al-Rahabah, proclaiming from over the minbar: I appeal to every Muslim who heard the Messenger of Allah (S), saying on the day of Ghadir Khumm. "â€˜Ali is the master of whomever I am his master", to stand up and give witness of what he heard, provided that he saw him by his own eyes and heard him by his own ears."