Amr bin Dinar narrates from Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) "gave judgment with an oath along with a witness."
Those whose narrations are not used as proof contradicted the two of them from Muhammad bin Muslim and added Tawus to its chain, and some narrated it by another route from Amr and added Jabir bin Zayd to its chain; but the narration of reliable narrators is not deemed defective because of the narration of weak ones.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20642