Sahih al-Bukhari, Hadith 649 ⓘ
Narrated `Abdullah bin `Umar: The reward of the congregational prayer is twenty seven times greater (than that of the prayer offered by a person alone).
قَالَ شُعَيْبٌ وَحَدَّثَنِي نَافِعٌ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، قَالَ تَفْضُلُهَا بِسَبْعٍ وَعِشْرِينَ دَرَجَةً.
also referenced under: Book 10, Hadith 46 (In-Book) & Volume 1, Book 11, Hadith 621 (USC-MSA)