Amr bin Shu'aib, from his father, from his grandfather, reports that the Prophet Muhammad (peace be upon him) said: "The burden of proof is upon the claimant, and an oath is required from the one who denies, except in the case of Qasamah, which has a different ruling."
Hadith Referenceسنن الدارقطني / كتاب فى الأقضية والأحكام وغير ذلك / 4508