Sahih Muslim Book 4, Hadith Number 1514.

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

Anas reported that when the Apostle of Allah (may peace be upon him) had to set out on a journey hurriedly, he delayed the noon prayer to the earlier time for the afternoon prayer, and then he would combine them, and he would delay the sunset prayer to the time when the twilight would disappear and then combine it with the ‘Isha prayer.