Sa'id bin Amr bin Shurahbil narrates from his father, and he from his grandfather, that he said: We found in the books of Sayyiduna Sa'd (may Allah be pleased with him): "The Messenger of Allah (peace and blessings be upon him) judged by one witness and an oath."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20662