Imam al-Shafi'i (may Allah have mercy on him) said: "Since the Messenger of Allah (peace and blessings be upon him) made Sayyiduna Rukanah (may Allah be pleased with him) swear an oath in the matter of divorce, this indicates that the oath in divorce is just as it is in other matters."
Ibn Abi Mulaykah said: Ibn Abbas (may Allah be pleased with them both) wrote to me that the Prophet (peace and blessings be upon him) "placed the oath upon the defendant."
(b) This covers every defendant, except where evidence establishes otherwise.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20727