Sahih Al Bukhari – Book of Call To Prayers (Adhaan)

Volumn 001, Book 011, Hadith Number 618.
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Narated By 'Abdullah bin Umar RA : Allah's Apostle PBUH said, "The prayer in congregation is twenty seven times superior to the prayer offered by person alone."

Sahih Al Bukhari – Book of Call To Prayers (Adhaan)

Volumn 001, Book 011, Hadith Number 619.
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Narated By Abu Said Al-Khudri RA : The Prophet PBUH said, "The prayer in congregation is twenty five times superior to the prayer offered by person alone."

And Allah knows the best.