Ibn Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "The proof is upon the claimant and the oath is upon the defendant (that is, the one against whom the claim is made)."
Abu al-Qasim (al-Tabarani) said: No one narrated it from Sufyan except al-Firyabi.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21203