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)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / Q18341