Amr bin Shu'ayb narrates from his father, from his grandfather (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said: "The evidence is upon the claimant and the oath is upon the defendant, except in qasamah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسامة / 16445