(17) باب صَلاَةِ اللَّيْلِ وَعَدَدِ رَكَعَاتِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي اللَّيْلِ وَأَنَّ الْوِتْرَ رَكْعَةٌ وَأَنَّ الرَّكْعَةَ صَلاَةٌ صَحِيحَةٌ
Abu Salama b. Abd al-Rahman asked 'A'isha about the (night) prayer of the Messenger of Allah (ﷺ) during the month of Ramadan. She said:
The Messenger of Allah (ﷺ) did not observe either in Ramadan or in other months more than eleven rak'ahs (of the night prayer). He (in the first instance) observed four rak'ahs. Ask not about their excellence and their length (i. e. these were matchless in perfection and length). He again observed four rak'ahs, and ask not about their excellence and their length. He would then observe three rak'ahs (of the Witr prayer). 'A'isha again said: I said: Messenger of Allah, do you sleep before observing the Witr prayer? He said: O 'A'isha, my eyes sleep but my heart does not sleep.
Reference: Sahih Muslim 738 a
In-book reference: Book 6, Hadith 152
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1607