Hisham bin Urwah reports from his father that Sayyidah Aishah (may Allah be pleased with her) used to say: The idle oath is one sworn in argument and in jest, and talk on which the heart is not set. The expiation is due for every oath that you swear in earnest, whether in anger or otherwise, that you will certainly do something or certainly leave it. That is the binding oath for which Allah Almighty has made expiation obligatory.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19935