Sahih Bukhari Volume 3, Book 48, Hadith 2515

حَدَّثَنَا خَلاَّدُ بْنُ يَحْيَى، حَدَّثَنَا نَافِعُ بْنُ عُمَرَ، عَنِ ابْنِ أَبِي مُلَيْكَةَ، قَالَ كَتَبْتُ إِلَى ابْنِ عَبَّاسٍ فَكَتَبَ إِلَىَّ أَنَّ النَّبِيَّ صلى الله عليه وسلم قَضَى أَنَّ الْيَمِينَ عَلَى الْمُدَّعَى عَلَيْهِ‏

Narrated By Ibn Abu Mulaika : I wrote a letter to Ibn 'Abbas and he wrote to me that the Prophet had given the verdict that the defendant had to take an oath.