Sunan an-Nasa'i, Volume 1, Book 6, Hadith 599 (DarusSalam) ⓘ
It was narrated from Ibn 'Umar that if the Messenger of Allah was in a hurry to travel, he would combine Maghrib and 'Isha'.
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ إِذَا جَدَّ بِهِ السَّيْرُ جَمَعَ بَيْنَ الْمَغْرِبِ وَالْعِشَاءِ .
also referenced under: Book 6, Hadith 605 (In-Book)