Sahih Bukhari Volume 3, Book 32, Hadith Number 230.

Narated By Abu Salama bin ‘Abdur Rahman : That he asked ‘Aisha “How was the prayer of Allah’s Apostle in Ramadan?” She replied, “He did not pray more than eleven Rakat in Ramadan or in any other month. He used to pray four Rakat… let alone their beauty and length… and then he would pray four… let alone their beauty and length… and then he would pray three Rakat (Witr).” She added, “I asked, ‘O Allah’s Apostle! Do you sleep before praying the Witr?’ He replied, ‘O ‘Aisha! My eyes sleep but my heart does not sleep.”