Abi Salamah bin 'Abdurrahmaan Radiyallahu 'Anhu says he asked 'Aayesha Radiyallahu 'Anha "How was the salaah (how many rak'ah tahajjud was performed) of Rasulullah Sallallahu Alayhi Wasallam in Ramadaan? "She replied:
"Rasulullah Sallallhu 'Alayhi Wasallam did not perform more than eleven rak'ahs during Ramadhaan or after Ramadhaan. (Eight rak'ahs tahajjud and three rak'ahs witr). He performed four rak'ahs. Do not ask of its length or how wonderfully (i.e. with humbleness and concentration) it was performed. In the same manner he performed four more rak'ahs. In the same manner he again performed four rak'ahs, and do not ask of its length or how wonderfully it was performed. After that he performed three rakahs witr". Aayesha Radiyallahu 'Anha says "I said: "O Messenger of Allah. Do you sleep before you perform witr?" He replied: "O Aayesha, my eyes sleep, but my heart remains awake". (This is a special gift to the ambiyah (prophets) that their hearts remain awake at all times)
English reference: Book 39, Hadith 257
Arabic reference: Book 40, Hadith 270