Sunan an-Nasa'i, Hadith 837 ⓘ
It was narrated from Ibn 'Umar that the Messenger of Allah said: "Praying in congregation is twenty-seven times better than praying alone."
أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " صَلاَةُ الْجَمَاعَةِ تَفْضُلُ عَلَى صَلاَةِ الْفَذِّ بِسَبْعٍ وَعِشْرِينَ دَرَجَةً " .
also referenced under: Book 10, Hadith 61 (In-Book) & Volume 1, Book 10, Hadith 838 (DarusSalam)