Abu Dawud Book 2, Hadith Number 1332.
Chapter : On the number of rakahs of the prayer at night.
Narated By N/A : This tradition has also been transmitted bu Ibn Shihab through a different chain of narrators to the same effect. This version adds: He would observe witr with a single rak’ah and 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 his call for the dawn prayer and the dawn became clear to him… Then the narrator transmitted the rest of the tradition to the same effect.