Sa'id bin Jubayr (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that declaring (one's wife) forbidden is an oath; he should give its expiation. And he said: "Indeed, in the Messenger of Allah you have an excellent example" [Surah al-Ahzab: 21].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15055