Musnad Ahmad 1153:2111

'Abd al-Samad narrated to us, he said: Fatimah bint 'Abd al-Rahman narrated to me, she said: My mother narrated to me, that she said: I asked 'Aishah, and her uncle had sent her, and he said: "One of your sons sends you greetings, and asks you about 'Uthman ibn 'Affan, for the people have insulted him." She said: "May Allah curse those who curse him. By Allah, he was sitting with the Prophet of Allah, may Allah bless him and grant him peace, and the Messenger of Allah, may Allah bless him and grant him peace, was leaning his back against me, while Gabriel was revealing the Qur'an to him, and he was saying to him: 'Write, O 'Uthaym.' Allah would not have placed him in that position except that he was noble to Allah and His Messenger."