Musnad Ahmad 1153:182

Abu Mu'awiyah narrated to us, Hisham bin 'Urwah narrated to us, from his father, from 'Aishah, who said: "When the Messenger of Allah (peace and blessings of Allah be upon him) grew stout and heavy, he would recite whatever Allah the Mighty and Majestic willed while he was sitting. Then, when thirty or forty verses remained of the surah, he would stand up and recite them, then he would prostrate."