Sahih Bukhari, Chapter: Call to Prayers (Adhaan)

Abu Huraira said, "I heard Allah's Apostle saying, 'The reward of a prayer in congregation is twenty five times greater than that of a prayer offered by a person alone. The angels of the night and the angels of the day gather at the time of Fajr prayer.' " Abu Huraira then added, "Recite the Holy Book if you wish, for "Indeed, the recitation of the Qur'an in the early dawn (Fajr prayer) is ever witnessed." (17.18). 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).