Abu Sinan al-Du'ali (may Allah have mercy on him) said that he visited Sayyiduna Ali (may Allah be pleased with him) during an illness of his and said: O Commander of the Believers! We fear for you in this ailment of yours (lest you be martyred in it). Sayyiduna Ali (may Allah be pleased with him) said: "By Allah! I have no fear for myself, because I have heard the Messenger of Allah (peace and blessings be upon him) say: 'Soon you will be struck here and here' — he (peace and blessings be upon him) was pointing to his temples — 'and your beard will be soaked with blood, and the one who does it will be the most wretched of people, just as the one who hamstrung the she-camel was the most wretched person among the people of Thamud.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16069