Musnad Ahmad 658:125

Yahya ibn Adam and Abu Ahmad narrated to us, they said: Isra'il narrated to us, from Abu Ishaq, from al-Bara', who said: A man came to the Prophet, peace and blessings of Allah be upon him, covered in iron [armor]. He said: "Shall I fight or embrace Islam?" He said: "Rather, embrace Islam, then fight." So he embraced Islam, then fought, and was killed. The Messenger of Allah, peace and blessings of Allah be upon him, said: "He did a little, and was rewarded much."