Musnad Ahmad 1153:1141

'Abd al-Rahman narrated to us, from Mu'awiyah, from 'Abdullah ibn Abi Qays, who said: I asked 'Aishah: "With how many [rak'ahs] did the Messenger of Allah, peace and blessings of Allah be upon him, perform the Witr?" She said: "With four and three, and six and three, and eight and three, and ten and three. He would not perform Witr with more than thirteen, nor less than seven. And he would not leave out the two rak'ahs [before the Witr]."