Sayyiduna Ali (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that the Prophet (peace and blessings be upon him) "judged by one witness and the oath of the one who has the right," and Sayyiduna Ali (may Allah be pleased with him) also gave the same judgement in Iraq.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20658