Ja'far bin Muhammad (may Allah have mercy on him) narrates from his father, and he from his grandfather, that the Messenger of Allah (peace and blessings be upon him) judged by an oath and a witness, and likewise Ali (may Allah be pleased with him) also used to judge in this way.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20656