Sahih al-Bukhari 3695 and Sahih Muslim 2403c preserve the strongest core: the Prophet Muhammad ﷺ gave Uthman ibn Affan glad tidings of Paradise together with notice of a future calamity or trial. A separate report in Jami al-Tirmidhi states more explicitly that Uthman would be killed wrongfully during a fitnah. Other reports warn him not to remove the 'garment' Allah would place upon him if people later pressured him to do so. Uthman was ultimately killed in Medina in 35 AH / 656 CE, forming the historical outcome layer of these earlier foretellings.
In Sahih al-Bukhari, Abu Musa al-Ash‘ari relates that the Prophet gave Uthman glad tidings of Paradise together with a calamity that would later befall him. Sahih Muslim preserves the same theme in the setting of the well of Aris, again combining the promise of Paradise with notice of a coming trial or turmoil.
Those two Sahih reports should be kept in their own wording. They foretell a future calamity, but they do not in these particular narrations explicitly say that Uthman would be martyred or wrongfully killed.
A separate report in Jami al-Tirmidhi supplies that more specific layer. In it, the Prophet mentions a fitnah and, referring to Uthman, says that he will be killed wrongfully during it. Other supporting reports add another form of forewarning. Jami al-Tirmidhi tells Uthman that if Allah clothes him with a “shirt,” he should not remove it when people demand that he do so. Sunan Ibn Majah gives a related version in which authority is placed upon Uthman and he is warned not to surrender the garment Allah has given him under pressure.
The historical outcome came years later. Uthman became caliph and, after mounting opposition and siege, was killed in Medina in 35 AH / 656 CE. The fact and broad chronology of his killing are well established, while the detailed political causes and allocation of blame remain disputed and should not be reduced to a single partisan narrative.
The extraordinary element in this account is therefore prophetic foreknowledge. The Prophet announced a future trial and, in a supporting report, Uthman’s wrongful killing before those events occurred. The most accurate classification is a revelatory sign or foretelling, while Uthman’s later assassination remains the historical outcome of that prior warning.
Conclusion
The source set strongly supports high-confidence inclusion as a revelatory sign or foretelling: the Prophet gave Uthman glad tidings of Paradise while warning of a future trial, and a supporting Tirmidhi report explicitly foretells his wrongful killing during a fitnah. Uthman's death in 35 AH / 656 CE forms the later historical outcome, while the prophetic foreknowledge remains the extraordinary core.