The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1610
Abu Salama is reported to have said. I came to 'A'isha. I said: O mother, inform me about the prayer of the Messenger of Allah (may peace be upon him). She said: His (night prayer) in Ramadan and (during other months) was thirteen rak'ahs at night including two rak'ahs of fajr.