Sahih al-Bukhari 6663
Narrated `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)
حَدَّثَنِي مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا يَحْيَى، عَنْ هِشَامٍ، قَالَ أَخْبَرَنِي أَبِي، عَنْ عَائِشَةَ ـ رضى الله عنها ‏{‏لاَ يُؤَاخِذُكُمُ اللَّهُ بِاللَّغْوِ‏}‏ قَالَ قَالَتْ أُنْزِلَتْ فِي قَوْلِهِ لاَ، وَاللَّهِ بَلَى وَاللَّهِ‏.‏