Abu Dawud Book 2, Hadith Number 1331.

Chapter : On the number of rakahs of the prayer at night.

Narated By ‘Aisha : Between the time when the Apostle of Allah (PBUH) finished the night prayer till the dawn broke, he used to pray eleven rak’ahs, uttering salutations at the end of every two and observing the witr with a single one, and during that he would make a prostration about as long as one of you would take to recite fifty verses before raising his head. When the mu’adhdhdin finished making the call for the dawn prayer, he stood up and prayed two rak’ahs; then he lay down on his right side till the mu’adhdhdin came to him.