Abu Dawud Book 2, Hadith Number 1329.
Chapter : On the number of rakahs of the prayer at night.
Narated By ‘Aisha : The Apostle of Allah (PBUH) used to pray ten rak’ahs during the night, and would observe the witr with one rak’ah; he then prayed two rak’ahs of the dawn prayer. Thus he prayed thirteen rak’ahs in all.