Musannaf Ibn Abi Shayba 3:4443

Hushaym narrated to us, from Abu Bishr, from Sa’id ibn Jubayr, who said: And Hushaym said: And Mansur informed us, from al-Hasan, both of them said: The Messenger of Allah (peace be upon him) used to perform Witr with nine rak'ahs. When he aged and became heavy, he performed Witr with seven [rak'ahs] and two rak'ahs while he was sitting.