It is narrated from Abu Sinan al-Du'ali that he visited Sayyiduna Ali (may Allah be pleased with him) during an illness of his. He says: I said to him: O Commander of the Believers! We were very worried and fearful for you in this illness of yours. He said: "But by Allah, I had no fear for my life from it, because I heard the truthful and believed Messenger of Allah (peace and blessings be upon him) say: 'You will indeed be struck here and here (and he pointed to his two temples), until blood flows from them and your beard is dyed with it, and the one who does this will be the most wretched of people, just as the one who hamstrung the she-camel was the most wretched of the people of Thamud.'" This hadith is sahih (authentic) on the condition of Imam Bukhari, but the two Shaykhs did not record it.