(13) بَابُ عَمَلٌ صَالِحٌ قَبْلَ الْقِتَالِ
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: Sahih al-Bukhari 2808
In-book reference: Book 56, Hadith 24
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 4, Book 52, Hadith 63