Musnad Ahmad 1153:524

Abu al-Mughirah narrated to us, al-Awza'i narrated to us, al-Zuhri narrated to me, from 'Urwah, from 'Aishah, she said: "The Prophet, peace and blessings of Allah be upon him, used to pray between the last 'Isha' and the breaking of the dawn eleven rak'ahs. He would perform the taslim after every two rak'ahs, and he would perform witr with one. He would remain in his prostration for the duration it takes one of you to recite fifty verses. When the mu'adhdhin fell silent, 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."