Abu Dawud Book 35, Hadith Number 4639.

Narated By Umar ibn al-Khattab : Al-Aqra’, the mu’adhdhin (announcer) of Umar ibn al-Khattab said: Umar sent me to a bishop and I called him.

Umar said to him: Do you find me in the Book? He said: Yes. He asked: How do you find me? He said: I find you (like a) castle. Then he raised a whip to him, saying: What do you mean by castle? He replied: An iron castle and severely trustworthy. He asked: How do you find the one who will come after me? He said: I find him a pious caliph, except that he will prefer his relatives. Umar said: May Allah have mercy on Uthman: He said it three times. He then asked: How do you find the one who will come after him?

He replied: I find him like rusty iron. Umar then put his hand on his head, and said: O filthy! O filthy! He said: Commander of the Faithful! He is a pious caliph, but when he is made caliph, the sword will be unsheathed and blood will be shed.