Language:
كتاب الأذان
10
Call to Prayers (Adhaan)
(31)
Chapter: Superiority of the Fajr (early morning) prayer in congregation.
(31)
باب فَضْلِ صَلاَةِ الْفَجْرِ فِي جَمَاعَةٍ
Sahih al-Bukhari 650

Narrated Salim:

I heard Um Ad-Darda' saying, "Abu Ad-Darda' entered the house in an angry mood. I said to him. 'What makes you angry?' He replied, 'By Allah! I do not find the followers of Muhammad doing those good things (which they used to do before) except the offering of congregational prayer." (This happened in the last days of Abu Ad-Darda' during the rule of `Uthman) .

حَدَّثَنَا عُمَرُ بْنُ حَفْصٍ، قَالَ حَدَّثَنَا أَبِي قَالَ، حَدَّثَنَا الأَعْمَشُ، قَالَ سَمِعْتُ سَالِمًا، قَالَ سَمِعْتُ أُمَّ الدَّرْدَاءِ، تَقُولُ دَخَلَ عَلَىَّ أَبُو الدَّرْدَاءِ وَهْوَ مُغْضَبٌ فَقُلْتُ مَا أَغْضَبَكَ فَقَالَ وَاللَّهِ مَا أَعْرِفُ مِنْ أُمَّةِ مُحَمَّدٍ صلى الله عليه وسلم شَيْئًا إِلاَّ أَنَّهُمْ يُصَلُّونَ جَمِيعًا‏.‏
Reference : Sahih al-Bukhari 650
In-book reference : Book 10, Hadith 47
USC-MSA web (English) reference : Vol. 1, Book 11, Hadith 622
  (deprecated numbering scheme)