Sahih Muslim , Book 4, Hadith 1508 (USC-MSA) ⓘ
Ibn 'Umar reported: When the Messenger of Allah was in a state of hurry on a journey, he combined the sunset and 'Isha' prayers.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا عَجِلَ بِهِ السَّيْرُ جَمَعَ بَيْنَ الْمَغْرِبِ وَالْعِشَاءِ .
also referenced under: Book 6, Hadith 52 (In-Book) & Hadith 703 (Reference)