Abu Dawud Book 2, Hadith Number 1354.
Chapter : On the number of rakahs of the prayer at night.
Narated By ‘Aisha : The Apostle of Allah (PBUH) used to pray thirteen rak’ahs, observing six rak’ahs in pairs including the two rak’ahs of the dawn prayer. He would observe witr with five rak’ahs. He sat only in the last of them.