Qais b. 'Ubad reported:
We said to 'Ammar: Was your fighting (on the side of 'Ali in the Battle of Siffin) a matter of your own choice or you got its hints from Allah's Messenger (ﷺ) for it, is likely for one to err in one's own discretion or was it because of any covenant that Allah's Messenger (ﷺ) got from you? He said: It was not because of any covenant that Allah's Messenger (ﷺ) got from us which he did get from other people, and he further said that Allah's Messenger (ﷺ) said:" In my Ummah." And I think that Hudhaifa reported to me and according to Ghundar (the words are) that he said: In my Ummah, there would be twelve hypocrites and they would not be admitted to Paradise and they would not smell its odour, until the camel would pass through a needle's hole. Dubaila (ulcer) would be enough to (torment them) -a kind of flame of Fire which would appear in their shoulders and it would protrude from their chest.
Reference: Sahih Muslim 2779 b
In-book reference: Book 51, Hadith 13
USC-MSA web (English) reference(deprecated numbering scheme): Book 38, Hadith 6689