Sunan Abi Dawud 1224
سنن أبي داود#1224SahihNarrated Ibn 'Umar:While travelling the Messenger of Allah (ﷺ) would pray voluntary prayer on his riding beast in whatever direction it turned; and he would observe witr prayer, but he did not offer the obligatory prayers upon it
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
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih Bukhari (1098) Sahih Muslim (700)
In-book reference: 4:27