by Imam Malik, translation by A'isha `Abdarahman at-Tarjumana and Ya`qub Johnson , Volume: Shortening the Prayer
Chapter: Shortening the Prayer in Travel
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 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. "
Yahya related to me from Malik from Yahya ibn Said that he said to Salim ibn Abdullah, "What is the latest you have seen your father delay maghrib while on a journey?" and Salim replied, "One time the sun set when we were at Dhat al-Jaysh and he prayed maghrib at al-Aqiq."