It was narrated that 'Aishah said:
The Messenger of Allah said: "O 'Uthman, if Allah places you in authority over this matter (as the caliph) some day and the hypocrites want to rid you of the garment with which Allah has clothed you (i.e., the position of caliph), do not take it off." He said that three times. (One of the narrators) Nu'man said: "I said to 'Aishah: 'What kept you from telling the people that?' She said: 'I was made to forget it.'"
English reference: Vol. 1, Book 1, Hadith 112
Arabic reference: Book 1, Hadith 117