Masruq narrates from Sayyidah Aishah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) took an oath of abstention (ila') from his wives and declared (them) forbidden to himself; then he (peace and blessings be upon him) made the forbidden lawful and made expiation for the oath.
Note: This strengthens the view of one who holds that the word "forbidden", used without qualification, is not an oath, a divorce or a zihar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15071