Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1609
Abu Salama reported that he asked 'A'isha about the prayer of the Messenger of Allah (may peace he upon him) (during the night). The rest of the hadith is the same but with this exception that he (the Holy Prophet) observed nine rak'ahs including Witr.