Abu Dawud Book 2, Hadith Number 1345.

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 during the night, observing the witr prayer with nine (or as she said). He used to pray two rak’ahs while sitting and pray two rak’ahs of the dawn prayer between the adhan and the iqamah.