Musnad Ahmad 962:176

Hassan ibn Musa narrated to us, Amara—meaning Ibn Zadhan—narrated to us, Abu Ghalib narrated to me, from Abu Umamah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to perform Witr with nine [rak'ahs]. When he grew older and his flesh increased, he performed Witr with seven [rak'ahs] and prayed two rak'ahs while he was sitting, and he recited in them "Idha zulzilat" (Surah Az-Zalzalah) and "Qul ya ayyuha al-kafirun" (Surah Al-Kafirun).