It is narrated from Saeed bin Amr bin Shurahbeel bin Saeed bin Saad bin Ubadah, from his father, and he from his grandfather, that he said: "We found written in the books of Saad (may Allah be pleased with him) that the Messenger of Allah (ﷺ) gave a judgment with an oath and one witness."
Hadith Referenceمسند الإمام الشافعي / كتاب القضاء والأحكام والدعاوى والبينات واليمين ومع الشاهد والأيمان والشهادات / 1712
Hadith Takhrijاخرجه الترمذى الاحكام، باب ما جاء في اليمين مع الشاهد (1343)۔