مُوَطَّأُ مَالِكٍ
Muwatta Malik 335
Shortening the Prayer
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ عَنْ عَلِيِّ بْنِ حُسَيْنٍ، أَنَّهُ كَانَ يَقُولُ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا أَرَادَ أَنْ يَسِيرَ يَوْمَهُ جَمَعَ بَيْنَ الظُّهْرِ وَالْعَصْرِ وَإِذَا أَرَادَ أَنْ يَسِيرَ لَيْلَهُ جَمَعَ بَيْنَ الْمَغْرِبِ وَالْعِشَاءِ .
Yahya related to me from Malik that he had heard that AIi ibn Husain used to say, "The Messenger of Allah, (ﷺ), 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."
Arabic/English book reference: Book 9, Hadith 7