Musnad Ahmad 1153:48

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from 'Urwah ibn al-Zubayr, from 'A'ishah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to pray eleven rak'ahs after the 'Isha' prayer. When morning came, he would pray two light rak'ahs, then he would lie down on his right side until the mu'adhdhin came to him to inform him of the prayer."