Sahih Bukhari – Hadith No : 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).

Hadith In Arabic Text :

قال شعيب وحدثني نافع، عن عبد الله بن عمر، قال تفضلها بسبع وعشرين درجة‏.‏

Chapter : Call to Prayers (Adhaan) – كتاب الأذان