Musnad Ahmad 1153:1301

Ibn Numayr and Rawh—the meaning being the same—said: Hanzalah narrated to us, from al-Qasim ibn Muhammad, who said—Rawh said: I heard al-Qasim ibn Muhammad saying: I heard Aisha saying: "The prayer of the Messenger of Allah, peace and blessings of Allah be upon him, at night was ten rak'ahs; he would perform witr with one prostration, and he would pray the two rak'ahs of Fajr, so that makes thirteen."