Sunan Abi Dawud Book 42, Hadith 61
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.
Abu Dawud said: Al-dafr means filth or evil smell.
The prophet (ﷺ) went out during the time of (treaty of) al-Hudaibiyyah. He then mentioned the rest of the tradition. He said : ‘Urwah b. Mas’ud then came to him and began to speak to the Prophet (ﷺ). Whenever he talk to him, he caught his beard ; and al-Mughirah b. Shu’bah was standing near the head of the Prophet (ﷺ) with a sword with him and a helmet on him. He then struck his hand with the handle of the sword, saying : Keep away your hand from his beard. ‘Urwah then raised his head and said : Who is this ? The prophet said : Al-Mughirah b. Shu’bah.
the best of my people is the generation in which I have been sent, then their immediate followers, then their immediate followers. Allah knows best whether he mentioned the third or not. After them will be people who will give testimony without being asked, who will make vows which they do not fulfill, who will be treacherous and not to be trusted, among whom fatness will appear.