Sahih Muslim , Book 4, Hadith 1365 (USC-MSA) ⓘ
Ibn Umar reported Allah's Messenger assaying: Prayer said in a congregation is twenty-seven degrees more excellent than prayer said by a single person.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " صَلاَةُ الْجَمَاعَةِ أَفْضَلُ مِنْ صَلاَةِ الْفَذِّ بِسَبْعٍ وَعِشْرِينَ دَرَجَةً " .
also referenced under: Book 5, Hadith 313 (In-Book) & Hadith 650 (Reference)