From Sahih Bukhari (Hadith 999):
Narrated Sa`id bin Yasar:I was going to Mecca in the company of `Abdullah bin `Umar and when I apprehended the approaching dawn, I dismounted and offered the witr prayer and then joined him. `Abdullah bin `Umar said, "Where have you been?" I replied, "I apprehended the approaching dawn so I dismounted and prayed the witr prayer." `Abdullah said, "Isn't it sufficient for you to follow the good example of Allah's Messenger (ﷺ)?" I replied, "Yes, by Allah." He said, "Allah's Messenger (ﷺ) used to pray witr on the back of the camel (while on a journey)
Reference: Bukhari 999