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." . 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."
Source: sunnah.com
Other translations
Arabic
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 .
sunnah.com