Sahih Bukhari Volume 2, Book 19, Hadith 1147

حَدَّثَنَا أَبُو الْوَلِيدِ، حَدَّثَنَا شُعْبَةُ،‏.‏ وَحَدَّثَنِي سُلَيْمَانُ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ أَبِي إِسْحَاقَ، عَنِ الأَسْوَدِ، قَالَ سَأَلْتُ عَائِشَةَ ـ رضى الله عنها ـ كَيْفَ صَلاَةُ النَّبِيِّ صلى الله عليه وسلم بِاللَّيْلِ قَالَتْ كَانَ يَنَامُ أَوَّلَهُ وَيَقُومُ آخِرَهُ، فَيُصَلِّي ثُمَّ يَرْجِعُ إِلَى فِرَاشِهِ، فَإِذَا أَذَّنَ الْمُؤَذِّنُ وَثَبَ، فَإِنْ كَانَ بِهِ حَاجَةٌ اغْتَسَلَ، وَإِلاَّ تَوَضَّأَ وَخَرَجَ‏

Narrated 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)."