(31) باب فَضْلِ صَلاَةِ الْفَجْرِ فِي جَمَاعَةٍ
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).
Reference: Sahih al-Bukhari 649
In-book reference: Book 10, Hadith 46
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 11, Hadith 621