Musnad Ahmad 1153:33

Muhammad ibn Fudayl narrated to us, from al-A’mash, from ‘Umarah ibn ‘Umayr, from Yahya ibn al-Jazzar, from ‘Aishah, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to perform Witr with nine [rak'ahs]. When he aged and became heavy, he performed Witr with seven [rak'ahs]."