Musnad Ahmad 1153:448

Husayn ibn Muhammad narrated to us, he said: Ibn Abi Dhi’b and Abu al-Nadr informed us, from Ibn Abi Dhi’b, from al-Zuhri, from ‘Urwah, from ‘A’ishah, who said: "The Prophet, peace and blessings of Allah be upon him, used to pray between the last ‘Isha’ prayer and the dawn, eleven rak’ahs. He would perform the taslim after every two, and he would perform witr with one. He would prostrate during his prayer for the duration it would take one of you to recite fifty verses before he would raise his head. When the mu’adhdhin would fall silent from his first adhan, he would stand and pray two light rak’ahs, then he would lie down on his right side until the mu’adhdhin came to him, and he would go out with him."