Sunan an-Nasa'i 490
سنن النسائي#490SahihIt was narrated from Salim that his father said:"The Messenger of Allah (ﷺ) used to pray voluntary prayers (Usabbih) while riding his mount, facing whatever direction it was facing, and he would pray Witr likewise, but he would not offer the prescribed prayers on it
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih - Agreed Upon
In-book reference: 5:43