Musnad Ahmad 1153:1328

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Qatadah, from al-Hasan, he said: Sa'd ibn Hisham informed me that he heard 'Aishah saying: "The Messenger of Allah (peace and blessings of Allah be upon him) used to perform Witr with nine rak'ahs, and two rak'ahs while he was sitting. Then, when he became weak, he performed Witr with seven [rak'ahs], and two rak'ahs while he was sitting."