Ata bin Abi Rabah said: I and Ubayd bin Umayr al-Laythi were with Aishah (may Allah be pleased with her), when Ubayd bin Umayr asked about the saying of Allah, "Allah will not take you to account for what is vain in your oaths" [Surah al-Baqarah: 225]. She said: A person swears according to what he knows, then the matter turns out not to be so; there is no expiation for that.
Umar bin Qays narrated it this way, and he is not strong.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19941