From Sahih 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 (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
Reference: Bukhari 2808