Musnad Ahmad 1153:2183

Muhammad ibn Isma'il ibn Abi Fudayk narrated to us, he said: al-Dahhak—meaning ibn 'Uthman—narrated to us, from 'Abdullah ibn 'Urwah, from his father, from 'Aishah, that she said: "The majority of the prayer of the Messenger of Allah, may Allah bless him and grant him peace, when he became heavy and gained weight, was while he was sitting."