Muwatta Malik Book 9, Hadith 9
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."
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 Salih ibn Kaysan from Urwa ibn az-Zubayr that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "The prayer was prescribed as two rakas, both when settled and when travelling. Then the travelling prayer was kept as it was, and an increase was made in the prayer when settled. "