Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1506
'Abdullah b. 'Amir b. Rabi'a has reported on the authority of his father that he had seen the Messenger of Allah (may peace be upon him) observing Nafl player at night on a journey on the back of his ride in whichever direction it turned its face.