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كتاب مواقيت الصلاة
9  
Times of the Prayers
(21)
Chapter: The time of the Isha prayer. If the people get together (pray earlier), and if they come late (delay it)
(21)
باب وَقْتِ الْعِشَاءِ إِذَا اجْتَمَعَ النَّاسُ أَوْ تَأَخَّرُوا
Sahih al-Bukhari 565

Narrated Muhammad bin `Amr:

We asked Jabir bin `Abdullah about the prayers of the Prophet (ﷺ) . He said, "He used to pray Zuhr prayer at midday, the `Asr when the sun was still hot, and the Maghrib after sunset (at its stated time). The `Isha was offered early if the people gathered, and used to be delayed if their number was less; and the morning prayer was offered when it was still dark. "

حَدَّثَنَا مُسْلِمُ بْنُ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ سَعْدِ بْنِ إِبْرَاهِيمَ، عَنْ مُحَمَّدِ بْنِ عَمْرٍو ـ هُوَ ابْنُ الْحَسَنِ بْنِ عَلِيٍّ ـ قَالَ سَأَلْنَا جَابِرَ بْنَ عَبْدِ اللَّهِ عَنْ صَلاَةِ النَّبِيِّ، صلى الله عليه وسلم فَقَالَ كَانَ يُصَلِّي الظُّهْرَ بِالْهَاجِرَةِ، وَالْعَصْرَ وَالشَّمْسُ حَيَّةٌ، وَالْمَغْرِبَ إِذَا وَجَبَتْ، وَالْعِشَاءَ إِذَا كَثُرَ النَّاسُ عَجَّلَ، وَإِذَا قَلُّوا أَخَّرَ، وَالصُّبْحَ بِغَلَسٍ‏.‏
Reference : Sahih al-Bukhari 565
In-book reference : Book 9, Hadith 42
USC-MSA web (English) reference : Vol. 1, Book 10, Hadith 540
  (deprecated numbering scheme)