كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
5  
The Book of Mosques and Places of Prayer
(42)
Chapter: The virtue of prayer in congregation, and clarifying the stern warning against staying away from it, and that it is fard kifayah
(42)
باب فَضْلِ صَلاَةِ الجَمَاعَةِ وَبَيَانِ التَّشْدِيدِ فِي التَّخَلُّفِ عَنْهَا ‏
Sahih Muslim 649 a

Abu Huraira reported Allah's Messenger (ﷺ) as saying:

Prayer said in a congregation is twenty-five degrees more excellent than prayer said by a single person.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنِ ابْنِ شِهَابٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ صَلاَةُ الْجَمَاعَةِ أَفْضَلُ مِنْ صَلاَةِ أَحَدِكُمْ وَحْدَهُ بِخَمْسَةٍ وَعِشْرِينَ جُزْءًا ‏"‏ ‏.‏
Reference : Sahih Muslim 649a
In-book reference : Book 5, Hadith 308
USC-MSA web (English) reference : Book 4, Hadith 1360
  (deprecated numbering scheme)