Musnad Ahmad 3:81

Abu al-Mughirah narrated to us, Arta’ah—meaning Ibn al-Mundhir—narrated to us, Abu ‘Awn al-Ansari informed me that ‘Uthman ibn ‘Affan said to Ibn Mas‘ud: "Will you desist from what has reached me about you?" He offered some excuse. ‘Uthman said: "Woe to you, I have heard and memorized, and it is not as you heard. The Messenger of Allah (ﷺ) said: 'A leader will be killed, and an usurper will usurp.' And I am the one who will be killed, not ‘Umar. ‘Umar was only killed by one person, but they will gather against me."