Musnad Ahmad 1153:1882

Abu Kamil and 'Affan narrated to us, they said: Hammad ibn Salamah narrated to us, from Qatadah, from al-Hasan, from Sa'd ibn Hisham, from 'Aishah, that the Prophet, peace and blessings of Allah be upon him, used to perform Witr with nine rak'ahs. When he became heavy and fleshy, he performed seven rak'ahs, then he performed two rak'ahs while he was sitting. 'Affan said: When he became fleshy and heavy.