Sahih Bukhari Volume 4, Book 52, Hadith Number 63.

Narated By 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 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).”