It is narrated from Mu'adh bin Abdur Rahman that he narrated from Ibn Abbas (may Allah be pleased with him) and another man whose name he mentioned (but I do not remember his name now), who was among the companions of the Prophet ﷺ, that the Messenger of Allah ﷺ gave a verdict based on an oath and one witness.
Hadith Referenceمسند الإمام الشافعي / كتاب القضاء والأحكام والدعاوى والبينات واليمين ومع الشاهد والأيمان والشهادات / 1710
Hadith Takhrijاسناده ضعيف جدا الا ان المتن صحيح اخرجه البيهقي : 10 / 168 - وفى المعرفة السنن والآثار له (5911)۔