Muwatta Malik · #330
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ إِذَا جَمَعَ الأُمَرَاءُ بَيْنَ الْمَغْرِبِ وَالْعِشَاءِ فِي الْمَطَرِ جَمَعَ مَعَهُمْ .
English
(with regards to above) Malik said, "I believe that was during rain." Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to join the prayer along with the amirs if they joined maghrib and isha in the rain