Sunan An-Nasai, Chapter: The Book of the Adhan (The Call to Prayer)

"Between the time when he finished Isha prayer and Fajr, the Prophet (saw) used to pray eleven Rakahs, saying the Taslim after each two Rakahs, then praying Witr as one Rakah. He would prostrate for as long as it takes one to recite fifty verses, then he would raise his head. When the Muadhdhin finished the call to Fajr prayer and he could see the dawn, he would pray two brief Rakahs, then he would lie down on his right side until the Mu;adhdhin came to ask permission to say the Iqamah, then he would go out with him." Some of these narrators (Ibn Abi Dhib, Yunus and Amr bin Al-Harith) added some phrases not mentioned by the others in the Hadith. (Sahih)