Musnad Abu Ya'la al-Mawsili 150:283

Ahmad ibn ‘Isa narrated to us, Bishr ibn Bakr narrated to us, al-Awza‘i narrated to me, al-Zuhri narrated to me, ‘Urwah ibn al-Zubayr narrated to me, ‘A’ishah narrated to me, she said: The Messenger of Allah (may Allah bless him and grant him peace) used to pray between the final night prayer (‘Isha’) until the dawn broke, eleven rak‘ahs. He would perform the taslim between every two, and he would perform witr with one. He would remain in his prostration for the duration that one of you would recite fifty verses. Then, when the first mu’adhdhin became silent for the dawn prayer, 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.