Muwatta Malik, Book 8, Hadith 288 (In-Book)

Yahya related to me from Malik from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said, "Prayer in congregation is better than the prayer of a man by himself by twenty-seven degrees."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ صَلاَةُ الْجَمَاعَةِ تَفْضُلُ صَلاَةَ الْفَذِّ بِسَبْعٍ وَعِشْرِينَ دَرَجَةً ‏" ‏ ‏.‏

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also referenced under: Book 8, Hadith 1 (USC-MSA)