Musnad Abu Ya'la al-Mawsili 150:146

'Abd al-A'la narrated to us, from 'Abd al-Rahman ibn Abi al-Zinad, from Hisham ibn 'Urwah, from his father, from 'A'ishah: That the prayer of the Messenger of Allah, may Allah bless him and grant him peace, at night was eight rak'ahs, besides the Witr.