Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

and this is the Hadith of Abu Bakr. "During the period after he finished prayer the Isha until the Fajr, the Prophet (saw) used to pray eleven Rakah, saying the Salam after each two Rak;ah and praying Witr with one Rakah. He would prostrate for as long as it takes anyone of you to recite fifty Verses before he would raise his head. When the Mu'adh-dhin fell silent after the first Adhan for the Subh prayer, he would get up and pray two brief Rakah." Sahih