The Messenger of Allah ﷺ ruled that the oath is upon the defendant.
حدثنا محمد بن سهل بن عسكر البغدادي حدثنا محمد بن يوسف حدثنا نافع بن عمر الجمحي عن عبد الله بن أبي مليكة
أن رسول الله ﷺ قضى أن اليمين على المدعى عليه
No references are available.
Agreed upon [by Bukhari and Muslim]. Bukhari also recorded it in "Pledges When Not Traveling," in the chapter "When the pledger and the pledgee, or similar parties, disagree, the burden of proof is on the claimant and the oath is on the one who denies the claim," hadith no. 2514, and Muslim, hadith no. 1711, from the hadith of Nafi bin Umar, with the same wording.