Musnad Ahmad 3:22

Abu Qatan narrated to us, Yunus—meaning Ibn Abi Ishaq—narrated to us, from his father, from Abu Salamah ibn Abd al-Rahman, who said: Uthman looked down from the palace while he was besieged and said: "I adjure by Allah those who witnessed the Messenger of Allah (peace and blessings of Allah be upon him) on the day of Hira, when the mountain shook and he kicked it with his foot, then said: 'Be still, Hira; there is upon you none but a Prophet, a Siddiq, or a martyr,' while I was with him." So, men testified to that for him. He said: "I adjure by Allah those who witnessed the Messenger of Allah (peace and blessings of Allah be upon him) on the day of the Pledge of Ridwan, when he sent me to the polytheists, to the people of Makkah, and said: 'This is my hand, and this is the hand of Uthman,' and he pledged on my behalf." So, men testified to that for him. He said: "I adjure by Allah those who witnessed the Messenger of Allah (peace and blessings of Allah be upon him) say: 'Who will expand this house for us in the mosque in exchange for a house in Paradise?' So I bought it with my own wealth and expanded the mosque with it." So, men testified to that for him. He said: "And I adjure by Allah those who witnessed the Messenger of Allah (peace and blessings of Allah be upon him) on the day of the Army of Hardship (Jaysh al-Usrah), when he said: 'Who will spend an expenditure today that will be accepted?' So I equipped half the army from my own wealth." He said: So, men testified to that for him. "And I adjure by Allah those who witnessed the well of Rumah, its water being sold to the wayfarer, so I bought it with my own wealth and made it free for the wayfarer." He said: So, men testified to that for him.