Sayyiduna Amr bin Hazm and Sayyiduna al-Mughirah bin Shu'bah (may Allah be pleased with them both) said: We were with the Messenger of Allah (peace and blessings be upon him) when two disputants (came); one of them had a witness with him. He (peace and blessings be upon him) took an oath from the one who had the witness and judged in his favour.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20664