Abu Sinan al-Du'ali (may Allah have mercy on him) said: I went to visit Sayyiduna Ali (may Allah be pleased with him) when he was suffering from an illness, and I said to him: "O Abu al-Hasan! We feared for you during this illness of yours." He said: "But by Allah! I have no fear for myself from it, for I heard the Truthful and the Believed (peace and blessings be upon him) say: 'You will be struck a blow here and a blow here' - and he (peace and blessings be upon him) pointed to his two temples - 'and its blood will flow until it dyes your beard, and the one who does it will be the most wretched (of this Ummah), just as the one who hamstrung the she-camel was the most wretched of Thamud.'"
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 172