Asim bin Umar bin Qatadah (may Allah have mercy on him) said: When the people met the enemy in battle on the day of Badr, Sayyiduna Awf bin Afra' bin al-Harith (may Allah be pleased with him) said: "O Messenger of Allah! At what deed of His servant does Allah Almighty smile?" He (peace and blessings be upon him) said: "When Allah Almighty sees His servant plunge into battle with his body unprotected." So Sayyiduna Awf (may Allah be pleased with him) took off his coat of mail, then advanced and fought until he was martyred.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18198