The Messenger of Allah (ﷺ) used to pray eleven rak'ahs (at night, observing the witr with one rak'ahs). When he finished it (the prayer), he would lie down on his right side.
Sunan Abi Dawud Book 5, Hadith 86
← Previous Hadith The Messenger of Allah (ﷺ) used to pray ten rak'ahs during the night, and would observe the witr with one rak'ah, he then prayed two rak'ahs of the dawn prayer. Thus he prayed thirteen rak'ahs in all.

Sunan Abi Dawud Book 5, Hadith 85 Next Hadith → 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

Sunan Abi Dawud Book 5, Hadith 87