Sa'id bin Jubayr (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that a man came to him and said: I have made my wife forbidden for myself. He said: You have lied; she is not forbidden for you. Then he recited this verse: "O Prophet, why do you forbid what Allah has made lawful for you?" [Surah al-Tahrim: 1]. Upon you is the heaviest expiation of an oath: freeing a slave. The hadith of Abu Nu'aym does not contain the words "upon you is the heaviest expiation." In one narration (from him) it is that this is a matter of choice (among the expiations).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15057