Sahih Muslim Book 4, Hadith Number 1511.

Chapter : Permissibility of saying Nafl prayer on a riding beast while on a journey, in whatever direction it turns.

Salim b. ‘Abdullah reported that his father had said: I saw the Messenger of Allah (may peace be upon him) delaying the sunset prayer till he would combine it with the ‘Isha when he hastened to set out on a journey.