Hadith 10:46
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
Original Reference: Sahih al-Bukhari 649