Muwatta Malik Book 9, Hadith 8
Yahya related to me from Malik that he had heard that AIi ibn
Husain used to say, "The Messenger of Allah, may Allah bless him and
grant him peace, would join dhuhr and asr if he wished to travel the
same day, and he would join maghrib and isha if he wished to travel
the same night."
Yahya related to me from Malik from Ibn Shihab that he had asked Salim ibn Abdullah, "Can you join dhuhr and asr when travelling?" He said, "Yes, there is no harm in that. Haven't you seen the people pray on Arafa?"
Yahya related to me from Malik from Ibn Shihab from a man of the family of Khalid ibn Asid that he said to Abdullah ibn Umar, "Abu Abd ar-Rahman, we find the fear prayer and the prayer when settled mentioned in the Qur'an, but we do not find any mention of the travelling prayer in it." Ibn Umar said, "Son of my brother! Allah the Mighty and Majestic sent us Muhammad, may Allah bless him and grant him peace, and we know nothing. We only do as we saw him doing."