Musnad Ahmad 1153:698

Hassan ibn Musa narrated to us, he said: Ibn Lahi'ah narrated to us, he said: Abu al-Aswad narrated to us, from 'Urwah, from 'Aishah, "That the Messenger of Allah, peace and blessings of Allah be upon him, used to pray thirteen prostrations (rak'ahs) at night, and the majority of his prayer was while standing. When he grew older and heavier, the majority of his prayer was while sitting. He would pray his prayer while I was lying across in front of him on the bedding upon which he slept, until he wished to perform the Witr, he would pinch me, so I would get up, and he would perform the Witr. Then he would lie down until he heard the call to prayer, then he would stand up, perform two light prostrations (rak'ahs), then press his side to the ground, then go out to the prayer."