Sahih al-Bukhari, Hadith 2808 ⓘ

Narrated Al-Bara: A man whose face was covered with an iron mask (i.e. clad in armor) came to the Prophet and said, "O Allah's Messenger! Shall I fight or embrace Islam first? "The Prophet said, "Embrace Islam first and then fight." So he embraced Islam, and was martyred. Allah's Messenger 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 ‏"‏‏.‏