Sahih al-Bukhari, Volume 1, Book 11, Hadith 618 (USC-MSA) ⓘ
Narrated `Abdullah bin `Umar: Allah's Messenger said, "The prayer in congregation is twenty seven times superior to the prayer offered by person alone."
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، قَالَ أَخْبَرَنَا مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " صَلاَةُ الْجَمَاعَةِ تَفْضُلُ صَلاَةَ الْفَذِّ بِسَبْعٍ وَعِشْرِينَ دَرَجَةً " .
also referenced under: Book 10, Hadith 42 (In-Book) & Hadith 645 (DarusSalam)