Musnad Ahmad 3:107

Al-Walid ibn Muslim narrated to us, Shu'ayb Abu Shaybah narrated to me, he said: I heard 'Ata' al-Khurasani saying: I heard Sa'id ibn al-Musayyib saying: I saw 'Uthman sitting in the seats, and he called for food that had been touched by fire, so he ate it. Then he stood for the prayer and prayed. Then 'Uthman said: I have sat in the seat of the Messenger of Allah, may Allah bless him and grant him peace, and I have eaten the food of the Messenger of Allah, and I have prayed the prayer of the Messenger of Allah, may Allah bless him and grant him peace.