Abu Dawud Book 2, Hadith Number 1330.
Chapter : On the number of rakahs of the prayer at night.
Narated By ‘Aisha, wife of the Prophet (PBUH) : The Apostle of Allah (PBUH) used to pray eleven rak’ahs (at night), observing the witr with one rak’ah. When he finished it (the prayer), he would lie down on his right side.