Language:
كتاب الأيمان والنذور
83
Oaths and Vows
(14)
Chapter: "Allah will not call you to account for that which is unintentional in your oaths ..."
(14)
بَابُ: {لاَ يُؤَاخِذُكُمُ اللَّهُ بِاللَّغْوِ فِي أَيْمَانِكُمْ وَلَكِنْ يُؤَاخِذُكُمْ بِمَا كَسَبَتْ قُلُوبُكُمْ وَاللَّهُ غَفُورٌ حَلِيمٌ}
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.‏
Reference : Sahih al-Bukhari 6663
In-book reference : Book 83, Hadith 41
USC-MSA web (English) reference : Vol. 8, Book 78, Hadith 656
  (deprecated numbering scheme)