Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1505
Salim 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.