Musnad Ahmad 1156:266

Abu Mu'awiyah narrated to us, from al-A'mash, from 'Amr ibn Murrah, from Yahya ibn al-Jazzar, from Umm Salamah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to perform Witr with thirteen [rak'ahs]. When he grew old and weak, he performed Witr with seven."