Sahih al-Bukhari 649
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · Superiority of the Fajr (early morning) prayer in congregation.
Narrated `Abdullah bin `Umar:
قَالَ شُعَيْبٌ وَحَدَّثَنِي نَافِعٌ ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ، قَالَ تَفْضُلُهَا بِسَبْعٍ وَعِشْرِينَ دَرَجَةً .
The reward of the congregational prayer is twenty seven times greater (than that of the prayer offered by a person alone).