Sahih Muslim Book 4, Hadith Number 1610.

Chapter : Permissibility of observing Nafl (voluntary prayer) standing or sitting and observing some part of it standing or sitting.

Abu Salama is reported to have said: I came to ‘Aisha. 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.