Sayyiduna al-Bara' (may Allah be pleased with him) said: A man covered in iron came to the Messenger of Allah (peace and blessings be upon him) and said: Shall I fight, or accept Islam? He (peace and blessings be upon him) said: "First accept Islam, then fight." So he accepted Islam, then fought and was martyred. He (peace and blessings be upon him) said: "The deed is little, and the reward given is much."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18542