مُوَطَّأُ مَالِكٍ
Muwatta Malik 331
Shortening the Prayer
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا عَجِلَ بِهِ السَّيْرُ يَجْمَعُ بَيْنَ الْمَغْرِبِ وَالْعِشَاءِ .
3 Yahya related to me from Malik from Nafi that Abdullah ibn Umar said, "The Messenger of Allah, (ﷺ), used to join maghrib and isha together when it was urgent to travel."
Arabic/English book reference: Book 9, Hadith 3