Amr bin Shu'ayb narrates from his father, and he from his grandfather (may Allah be pleased with them both), that the Prophet (peace and blessings be upon him) "judged by an oath and a single witness."
Mutarrif bin Mazin and Muhammad bin Abdullah bin Umayr are both not strong, and this (narration), through its being mursal, is a supporting witness for what has preceded.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20669