Sahih Muslim 1758
صحيح مسلم#1758SahihIbn Umar reported Allah's Messenger (ﷺ) as saying:Witr is a rak'ah at the end of the night prayer
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 6:187