Musnad Ahmad 1153:1087

Yazid narrated to us, he said: Ibn Abi Dhi'b informed us, from al-Zuhri, from 'Urwah, from 'Aishah, she said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to pray between the time he finished the 'Isha prayer until 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 finished the 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, then he would go out with him."