عَنِ ابْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا عَنِ
النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «لَوْ يُعْطَى النَّاسُ بِدَعْوَاهُمْ لَادَّعَى نَاسٌ دِمَاءَ رِجَالٍ وَأَمْوَالَهُمْ وَلَكِنَّ الْيَمِينَ عَلَى الْمُدَّعَى عَلَيْهِ»
The Prophet (sallallahu alayhi wa sallam) said:, “If people were given what they asked when they brought a case some would claim the lives and property of others; but the oath must be taken by the defendant.” Muslim transmitted it.
In the commentary on his work by Nawawi it says that in Baihaqi’s version with an isnad which is hasan or sahih there is an addition on the authority of Ibn Abbas which is traced to the Prophet (sallallahu alayhi wa sallam), “But the proof lies on the plaintiff and the oath must be taken by him who rejects the claim.”