It is narrated from Ibn Abbas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The burden of proof is upon the claimant." (The narrator says) I think he also said, though I do not remember it with certainty, that he (the Prophet) also said: "(In the absence of witnesses) the oath is upon the defendant."
Hadith Referenceمسند الإمام الشافعي / كتاب القضاء والأحكام والدعاوى والبينات واليمين ومع الشاهد والأيمان والشهادات / 1693
Hadith Takhrijاخرجه البخاري، التفسير ، باب ﴿ان الذين يشترون بعهد الله ....﴾ (4552) ومسلم، الأقضية، باب اليمين على المدعى عليه (1711)۔