Amr bin Shu'aib, from his father, from his grandfather, narrates 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, for which the ruling is different."
Hadith Referenceسنن الدارقطني / كتاب الحدود والديات وغيره / 3191