Sunan An-Nasai, Chapter: The Book of the Times (of Prayer)

"We came back with Ibn Umar from Makkah. One night he kept on travelling until evening came, and we thought that he had forgotten the prayer. We said to him: 'The prayer!' But he kept quiet and kept going until the twilight had almost disappeared, then he stopped and prayed, and when the twilight disappeared he prayed Isha. Then he turned to us and said: This is what we used to do with the Messenger of Allah (saw) if he was in a hurry to travel.'" (Sahih)