Musnad Ahmad 1153:433

Ishaq ibn Isa narrated to us, he said: Malik narrated to me, from Sa’id ibn Abi Sa’id al-Maqburi, that Abu Salamah ibn Abd al-Rahman informed him, he said: I asked Aisha: "How was the prayer of the Messenger of Allah, peace and blessings of Allah be upon him, in Ramadan?" She said: "He would not increase in Ramadan, nor in other than it, upon eleven rak'ahs. He would pray four, so do not ask about their length and their beauty. Then he would pray four, so do not ask about their beauty and their length. Then he would pray three." Aisha said: I said: "O Messenger of Allah, do you sleep before you perform the witr?" He said: "O Aisha, my eyes sleep, but my heart does not sleep."