Narrated Ibn 'Abbas (RA):

The Prophet (ﷺ) said: "If people were given whatever they claimed (in disputes), some people would claim the lives and wealth of others; but the oath (of denial) must be taken by the defendant." [Agreed upon]. al-Baihaqi has reported with a Sahih (authentic) chain of narrators: "But the proof (Bayyinah) lies on the one who is making the claim, and the other (Yamin) must be taken by the one who rejects the claim."

عَنِ ابْنِ عَبَّاسٍ رَضِيَ اَللَّهُ عَنْهُمَا; أَنَّ اَلنَّبِيَّ ‏- صلى الله عليه وسلم ‏-قَالَ: { "لَوْ يُعْطَى اَلنَّاسُ بِدَعْوَاهُمْ, لَادَّعَى نَاسٌ دِمَاءَ رِجَالٍ, وَأَمْوَالَهُمْ, وَلَكِنِ اَلْيَمِينُ عَلَى اَلْمُدَّعَى عَلَيْهِ" } مُتَّفَقٌ عَلَيْه ِ [1]‏ وَلِلْبَيْهَقِيِّ بِإِسْنَادٍ صَحِيحٍ: { "اَلْبَيِّنَةُ عَلَى اَلْمُدَّعِي, وَالْيَمِينُ عَلَى مَنْ أَنْكَرَ } [2]‏ .‏