Maliks Muwatta Book 009, Hadith Number 007.

Section : Joining Two Prayers when Settled and when Travelling.

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."