Sunan Abi Dawud Book 5, Hadith 88
He would observe witr with a single rak'ah and make a prostration as long as you would take to recite fifty verses before raising his head. When the mu'adhdhin 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.
Some narrators added something more in their version.
Between the time when the Messenger of Allah (ﷺ) finished the night prayer till the dawn broke, he used to pray eleven rak'ahs, uttering the salutation 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 a one of you would take to recite fifty verses before raising his head. When the mu'adhdhin finished making the call for the dawn prayer, he stood up and prayed two short rak'ahs, then he lay down on his right side till the mu'adhdhin came to him
The Messenger of Allah (ﷺ) used to pray thirteen rak'ahs during the night, observing a witr out of that with five, he did not sit during the five except the last and then gave the salutation.
Abu Dawud said: Ibn Numair reported it from Hisham recently.