Hanash said that Sayyiduna Ali (may Allah be pleased with him) used to require witnesses along with the oath as well.
(b) Hanash narrates from Sayyiduna Ali (may Allah be pleased with him) that Sayyiduna Ali (may Allah be pleased with him) held this view when the proofs conflicted.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الدعوى والبينات / 21248