Umayr bin Rabi'ah said that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) sent for Ka'b al-Ahbar and said: O Ka'b! What description of me do you find (in the scriptures)? He said: I find your description to be a horn of iron. He asked: What is a horn of iron? He said: A sound and upright ruler who, in the matter of Allah, pays no heed to the blame of any blamer. He asked: Then what? He said: Then after you there will be a caliph whom a wrongdoing group will kill. He asked: Then what? He said: Then there will be tribulation (and trial).
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 119