Sahih Bukhari | Volume 4 | Hadith Number 769
Narrated by Abu Salama bin 'Abdur-Rahman
That he asked 'Aisha "How was the prayer of Allah's Apostle in the month of Ramadan?" She replied, "He used not to pray more than eleven Rakat whether in Ramadan or in any other month. He used to offer four Rakat, let alone their beauty and length, and then four Rakat, let alone their beauty and length. Afterwards he would offer three Rakat. I said, 'O Allah's Apostle! Do you go to bed before offering the Witr prayer?' He said, 'My eyes sleep, but my heart does not sleep.'"