Muwatta Malik 328

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 ‏.‏

Yahya related to me from Malik from Nafi that Abdullah ibn Umar said, "The Messenger of Allah, may Allah bless him and grant him peace, used to join maghrib and isha together when it was urgent to travel

Reference

Muwatta Malik 328

In-book

Book 9, Hadith 3