Ash'ath (may Allah have mercy on him) narrates from al-Hasan (may Allah have mercy on him) concerning (saying) "forbidden": if he intended an oath, an oath is meant; and if he intended divorce, it will be a divorce.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15063