Maliks Muwatta Book 9, Hadith Number 10.
Section : Shortening the Prayer in Travel.
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.”