Translation
(10) Call to Prayers (Adhaan)
كتاب الأذان
(31) Chapter: Superiority of the Fajr (early morning) prayer in congregation.
(31) باب فَضْلِ صَلاَةِ الْفَجْرِ فِي جَمَاعَةٍ
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.‏
Reference : Sahih al-Bukhari 649
In-book reference : Book 10, Hadith 46
USC-MSA web (English) reference : Vol. 1, Book 11, Hadith 621
  (deprecated numbering scheme)