Sahih al-Bukhari 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.‏