Sahih Bukhari, Chapter: Prayer at Night (Tahajjud)

I asked 'Aisha, "How is the prayer of Allah's Apostle during the month of Ramadan." She said, "Allah's Apostle never exceeded eleven Rakat in Ramadan or in other months; he used to offer four Rakat-- do not ask me about their beauty and length, then four Rakat, do not ask me about their beauty and length, and then three Rakat." Aisha further said, "I said, 'O Allah's Apostle! Do you sleep before offering the Witr prayer?' He replied, 'O 'Aisha! My eyes sleep but my heart remains awake'!"