Sahih Bukhari Volume 8, Book 78, Hadith Number 656.

Narated By ‘Aisha : Regarding: ‘Allah will not call you to account for that which is unintentional in your oaths…’ (2.225) This Verse was revealed concerning such oath formulas as: ‘No, by Allah!’ and ‘Yes, by Allah!’ something against his oath due to forgetfulness should he make expiation?). And the Statement of Allah: ‘And there is no blame on you if you make a mistake therein.’ (33.5) And Allah said: ‘(Moses said to Khadir): Call me not to account for what I forgot.” (18.73)