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)۔
It is narrated from Jabir bin Abdullah (may Allah be pleased with them both) that two men claimed ownership of an animal, and each of them presented testimony that the animal belonged to him and that he had raised (and bred) it. So the Messenger of Allah (ﷺ) decided in favor of the one in whose possession the animal was.
Hadith Referenceمسند الإمام الشافعي / كتاب القضاء والأحكام والدعاوى والبينات واليمين ومع الشاهد والأيمان والشهادات / 1694
Hadith Takhrijاسناده ضعيـف جــدا فـان ابـن ابــى يحيى واسحاق بن أبي فروة متروكان اخرجه البيهقي : 10/ 256 ـ وفي المعرفة السنن والآثار له (5984) - والدارقطني : 20/4 - والبغوى (2504)۔