Sahih Muslim 1618
صحيح مسلم#1618SahihSalim b. 'Abdullah reported on the authority of his father that the Messenger of Allah (may peace be. upon him) used to observe Nafl (supererogatory) prayer on his ride no matter in what direction it turned its face, and he observed Witr too on it, but did not observe obligatory prayer on it
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: 6:49