Abu Dawud Book 2, Hadith Number 1203.
Chapter : On the combination of two prayers at a time.
Narated By Abdullah ibn Umar : Ibn Umar was informed about the death of Safiyyah (the wife of the Prophet) when he was at Mecca. He proceeded till the sun set and the stars shined. He said: When the Prophet (PBUH) was in a hurry about something while on a journey, he would combine both these prayers. He proceed till twilight had disappeared. He then combined both of them (the prayers).