Sahih Bukhari Volume 4, Book 56, Hadith 2809

حَدَّثَنِى 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 ‏"

Narrated By Al-Bara : A man whose face was covered with an iron mask (i.e. clad in armour) came to the Prophet and said, "O Allah's Apostle! 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 Apostle said, A Little work, but a great reward. "(He did very little (after embracing Islam), but he will be rewarded in abundance)."