Ata narrates from Sayyidah Aishah (may Allah be pleased with her) that the expiation for making one's wife forbidden for oneself is that of an oath.
(b) Sa'id bin Abi Arubah also speaks of giving the expiation of an oath.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15060