Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1509
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.