Imam al-Shafi'i (may Allah have mercy on him) said: "There is no harm in single combat (mubarazah). On the day of the Battle of Badr, Sayyiduna Ubaydah, Sayyiduna Hamzah and Sayyiduna Ali (may Allah be pleased with them) engaged in single combat by the order of the Prophet (peace and blessings be upon him)."
Qays bin Abbad (may Allah have mercy on him) said: I heard Abu Dharr (may Allah be pleased with him) swearing an oath that this verse, "These are two disputants who disputed concerning their Lord" [Surah al-Hajj: 19], was revealed about those who engaged in single combat on the day of Badr: Hamzah (may Allah be pleased with him), Ali (may Allah be pleased with him) and Ubaydah bin al-Harith (may Allah be pleased with him), and Utbah and Shaybah, the two sons of Rabi'ah, and al-Walid bin Utbah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18341