Sunan Abi Dawud 1335
سنن أبي داود#1335SahihNarrated 'Aishah, wife of Prophet (ﷺ):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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Zubair Ali ZaiSahih Muslim (736)
In-book reference: 5:86