Sahih Bukhari Volume 001, Book 003, Hadith Number 117.
Narated By Ibn 'Abbas : I stayed overnight in the house of my aunt Maimuna bint Al-Harith (the wife of the Prophet) while the Prophet was there with her during her night turn. The Prophet offered the 'Isha' prayer (in the mosque), returned home and after having prayed four Rakat, he slept. Later on he got up at night and then asked whether the boy (or he used a similar word) had slept? Then he got up for the prayer and I stood up by his left side but he made me stand to his right and offered five Rakat followed by two more Rakat. Then he slept and I heard him snoring and then (after a while) he left for the (Fajr) prayer.