Sunan Ibn Majah, Book 4, Hadith 55 (In-Book) ⓘ
It was narrated that Ibn 'Umar said: "The Messenger of Allah said: 'The prayer of a man in congregation is twenty-seven levels more virtuous than a man's prayer on his own.'"
حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ عُمَرَ، رُسْتَهْ حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عُمَرَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " صَلاَةُ الرَّجُلِ فِي جَمَاعَةٍ تَفْضُلُ عَلَى صَلاَةِ الرَّجُلِ وَحْدَهُ بِسَبْعٍ وَعِشْرِينَ دَرَجَةً " .
also referenced under: Hadith 789 (Reference) & Volume 1, Book 4, Hadith 789 (DarusSalam)