Sahih Bukhari Volume 002, Book 021, Hadith Number 247.
Narated By Al-Aswad : I asked 'Aisha "How is the night prayer of the Prophet?" She replied, "He used to sleep early at night, and get up in its last part to pray, and then return to his bed. When the Muadh-dhin pronounced the Adhan, he would get up. If he was in need of a bath he would take it; otherwise he would perform ablution and then go out (for the prayer)."