Sahih al-Bukhari 1098
صحيح البخاري#1098SahihNarrated Salim:At night `Abdullah bin `Umar used to offer the prayer on the back of his animal during the journey and never cared about the direction he faced. Ibn `Umar said, "Allah's Messenger (ﷺ) used to offer the optional prayer on the back of his Mount facing any direction and also used to pray the witr on it but never offered the compulsory 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 al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 18:18