Ja'far bin Muhammad narrates from his father, and he from Ali (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) "judged by a witness and an oath," and likewise Ali (may Allah be pleased with him) judged in Iraq.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20657