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كتاب الجهاد والسير
56  
Fighting for the Cause of Allah (Jihaad)
(13)
Chapter: Practising good deeds before taking part in a battle
(13)
بَابُ عَمَلٌ صَالِحٌ قَبْلَ الْقِتَالِ
Sahih al-Bukhari 2808

Narrated Al-Bara:

A man whose face was covered with an iron mask (i.e. clad in armor) came to the Prophet (ﷺ) and said, "O Allah's Messenger (ﷺ)! Shall I fight or embrace Islam first? "The Prophet (ﷺ) said, "Embrace Islam first and then fight." So he embraced Islam, and was martyred. Allah's Messenger (ﷺ) said, A Little work, but a great reward. "(He did very little (after embracing Islam), but he will be rewarded in abundance).

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ الرَّحِيمِ، حَدَّثَنَا شَبَابَةُ بْنُ سَوَّارٍ الْفَزَارِيُّ، حَدَّثَنَا إِسْرَائِيلُ، عَنْ أَبِي إِسْحَاقَ، قَالَ سَمِعْتُ الْبَرَاءَ ـ رضى الله عنه ـ يَقُولُ أَتَى النَّبِيَّ صلى الله عليه وسلم رَجُلٌ مُقَنَّعٌ بِالْحَدِيدِ فَقَالَ يَا رَسُولَ اللَّهِ أُقَاتِلُ وَأُسْلِمُ‏.‏ قَالَ ‏"‏ أَسْلِمْ ثُمَّ قَاتِلْ ‏"‏‏.‏ فَأَسْلَمَ ثُمَّ قَاتَلَ، فَقُتِلَ، فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ عَمِلَ قَلِيلاً وَأُجِرَ كَثِيرًا ‏"‏‏.‏
Reference : Sahih al-Bukhari 2808
In-book reference : Book 56, Hadith 24
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 63
  (deprecated numbering scheme)