Sayyiduna Zayd bin Thabit (may Allah be pleased with him) narrates from the Messenger of Allah (peace and blessings be upon him) that the Messenger of Allah (peace and blessings be upon him) said: "When the one making the demand has no proof, the oath is upon the one from whom it is demanded."
The hadith "The proof is upon the claimant, and the oath is upon the defendant" has been narrated to us through other routes, all of them weak, and what we have mentioned suffices.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21206