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كتاب الجمعة
11  
Friday Prayer
(33)
Chapter: When the Imam is delivering the Khutba, a light two Rak'a Salat (prayer) (Tahayyat-ul Masjid)
(33)
باب مَنْ جَاءَ وَالإِمَامُ يَخْطُبُ صَلَّى رَكْعَتَيْنِ خَفِيفَتَيْنِ
Sahih al-Bukhari 931

Narrated Jabir:

A man entered the Mosque while the Prophet (ﷺ) was delivering the Khutba. The Prophet (ﷺ) said to him, "Have you prayed?" The man replied in the negative. The Prophet (ﷺ) said, "Pray two rak`at."

حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عَمْرٍو، سَمِعَ جَابِرًا، قَالَ دَخَلَ رَجُلٌ يَوْمَ الْجُمُعَةِ وَالنَّبِيُّ صلى الله عليه وسلم يَخْطُبُ فَقَالَ ‏"‏ أَصَلَّيْتَ ‏"‏‏.‏ قَالَ لاَ‏.‏ قَالَ ‏"‏ فَصَلِّ رَكْعَتَيْنِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 931
In-book reference : Book 11, Hadith 55
USC-MSA web (English) reference : Vol. 2, Book 13, Hadith 53
  (deprecated numbering scheme)