The burden of proof is on the claimant

عَنْ ابْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا أَنَّ رَسُولَ اللَّهِ صلى الله عليه و سلم قَالَ: "لَوْ يُعْطَى النَّاسُ بِدَعْوَاهُمْ لَادَّعَى رِجَالٌ أَمْوَالَ قَوْمٍ وَدِمَاءَهُمْ، لَكِنَّ الْبَيِّنَةَ عَلَى الْمُدَّعِي، وَالْيَمِينَ عَلَى مَنْ أَنْكَرَ" . حَدِيثٌ حَسَنٌ، رَوَاهُ الْبَيْهَقِيّ [في"السنن" 10/252]، وَغَيْرُهُ هَكَذَا، وَبَعْضُهُ فِي "الصَّحِيحَيْنِ

On the authority of Ibn Abbas (may Allah be pleased with him), that the Messenger of Allah (peace and blessings of Allah be upon him) said: Were people to be given everything that they claimed, men would [unjustly] claim the wealth and lives of [other] people. But, the onus of proof is upon the claimant, and the taking of an oath is upon him who denies. A hasan hadeeth narrated by al-Baihaqee and others in this form, and part of it is in the two Saheehs

حَدَّثَنِي أَبُو الطَّاهِرِ، أَحْمَدُ بْنُ عَمْرِو بْنِ سَرْحٍ أَخْبَرَنَا ابْنُ وَهْبٍ، عَنِ ابْنِ جُرَيْجٍ، عَنِ ابْنِ أَبِي مُلَيْكَةَ، عَنِ ابْنِ عَبَّاسٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ ‏ "‏ لَوْ يُعْطَى النَّاسُ بِدَعْوَاهُمْ لاَدَّعَى نَاسٌ دِمَاءَ رِجَالٍ وَأَمْوَالَهُمْ وَلَكِنَّ الْيَمِينَ عَلَى الْمُدَّعَى عَلَيْهِ ‏"‏ ‏.‏

Ibn Abbas reported Allah's Apostle (ﷺ) as saying:If the people were given according to their claims, they would claim the lives of persons and their properties, but the oath must be taken by the defendant

حَدَّثَنَا نَصْرُ بْنُ عَلِيِّ بْنِ نَصْرٍ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ دَاوُدَ، عَنِ ابْنِ جُرَيْجٍ، عَنِ ابْنِ أَبِي مُلَيْكَةَ، أَنَّ امْرَأَتَيْنِ، كَانَتَا تَخْرِزَانِ فِي بَيْتٍ ـ أَوْ فِي الْحُجْرَةِ ـ فَخَرَجَتْ إِحْدَاهُمَا وَقَدْ أُنْفِذَ بِإِشْفًى فِي كَفِّهَا، فَادَّعَتْ عَلَى الأُخْرَى، فَرُفِعَ إِلَى ابْنِ عَبَّاسٍ، فَقَالَ ابْنُ عَبَّاسٍ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ لَوْ يُعْطَى النَّاسُ بِدَعْوَاهُمْ لَذَهَبَ دِمَاءُ قَوْمٍ وَأَمْوَالُهُمْ ‏"‏‏.‏ ذَكِّرُوهَا بِاللَّهِ وَاقْرَءُوا عَلَيْهَا ‏{‏إِنَّ الَّذِينَ يَشْتَرُونَ بِعَهْدِ اللَّهِ‏}‏‏.‏ فَذَكَّرُوهَا فَاعْتَرَفَتْ، فَقَالَ ابْنُ عَبَّاسٍ قَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ الْيَمِينُ عَلَى الْمُدَّعَى عَلَيْهِ ‏"‏‏.‏

Narrated Ibn Abu Mulaika:Two women were stitching shoes in a house or a room. Then one of them came out with an awl driven into her hand, and she sued the other for it. The case was brought before Ibn `Abbas, Ibn `Abbas said, "Allah's Messenger (ﷺ) said, 'If people were to be given what they claim (without proving their claim) the life and property of the nation would be lost.' Will you remind her (i.e. the defendant), of Allah and recite before her:--"Verily! Those who purchase a small gain at the cost of Allah's Covenant and their oaths..."(3.77) So they reminded her and she confessed. Ibn `Abbas then said, "The Prophet (ﷺ) said, 'The oath is to be taken by the defendant (in the absence of any proof against him)