Ibn Abbas wrote that the Prophet ﷺ ruled that the oath is upon the defendant.
كتب ابن عباس رضي الله عنهما: أن النبي ﷺ قضى باليمين على المدعى عليه.
Bukhari narrated it in the Book of Testimonies (2668), and Muslim narrated it in the Book of Judgments (2:1711), both of them via Nafi, from Ibn Abu Mulayka, who mentioned it.