Musannaf Ibn Abi Shayba 3:4441

Abu Bakr narrated to us, he said: Muhammad ibn Fudayl narrated to us, from al-A’mash, from ‘Umarah ibn ‘Umayr, from Yahya ibn al-Jazzar, from ‘Aishah, she said: The Messenger of Allah (may Allah bless him and grant him peace) used to perform Witr with nine [rak'ahs]. When he aged and became heavy, he performed Witr with seven.