Imam al-Shafi'i (may Allah have mercy on him) said: "Single combat (mubarazah) against the enemy took place in the presence of the Messenger of Allah (peace and blessings be upon him), and on the day of Badr a man of the Ansar, without armour, charged at a group of the polytheists after the Prophet (peace and blessings be upon him) had informed him of the good (and reward) in that (deed), and he was martyred."
The Shaykh (Imam al-Bayhaqi) (may Allah have mercy on him) said: "He was Sayyiduna Awf bin Afra' (may Allah be pleased with him), as Ibn Ishaq mentioned from Asim bin Umar bin Qatadah; and this (incident) will come in its proper place, if Allah wills, along with the mention of those who engaged in single combat before him (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / Q17915