Sahih Muslim Book 4, Hadith Number 1509.

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

Nafi’ reported that when Ibn ‘Umar was in a state of hurry on a journey, he combined the sunset and ‘Isha prayers after the twilight had disappeared, and he would say that when the Messenger of Allah (may peace be upon him) was in a state of hurry on a journey, he combined the sunset and ‘Isha prayers.