Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Salah (Prayers)

Abu Salamah asked Sayyidah Aishah about the salah of Allah's Messenger (SAW) (in the night) in Ramadan. She said, "Allah's Messenger (SAW) did not exceed eleven rakat in Ramadan or otherwise. He prayed four, but do not ask about their beauty and length. Then he (saw) prayed four, and do not ask about their beauty and length. Then he prayed three, and I said, "0 Messenger of Allah! Do you sleep before offering the witr?" He said, "My eyes sleep but the heart keeps awake." [Ahmed 24500, Bukhari 1147, Muslim 738, Abu Dawud 1341, Nisai 1693]