Sayyiduna Ash'ath bin Qays (may Allah be pleased with him) said: There was a dispute between me and a man. We came to the Messenger of Allah (peace and blessings be upon him). He (peace and blessings be upon him) asked: "Do you have proof?" I said: No. He (peace and blessings be upon him) said: "Then it is the other's oath."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الدعوى والبينات / 21219