The Book of Oaths
27/3. Chapter:
It is recommended for the one who swears an oath then sees that something else is better than it; To do that which is better and offer expiation for his oath
Sahih Muslim
وَحَدَّثَنَا شَيْبَانُ بْنُ فَرُّوخَ، حَدَّثَنَا الصَّعْقُ، - يَعْنِي ابْنَ حَزْنٍ - حَدَّثَنَا مَطَرٌ الْوَرَّاقُ، حَدَّثَنَا زَهْدَمٌ الْجَرْمِيُّ، قَالَ دَخَلْتُ عَلَى أَبِي مُوسَى وَهُوَ يَأْكُلُ لَحْمَ دَجَاجٍ وَسَاقَ الْحَدِيثَ بِنَحْوِ حَدِيثِهِمْ وَزَادَ فِيهِ قَالَ " إِنِّي وَاللَّهِ مَا نَسِيتُهَا " .
Zahdam al-Jarmi reported: I visited Abu Musa and lie was eating fowl's meat. The rest of the hadith is the same with this addition that he (the Holy Prophet) said: By Allah, I did not forget it.
Reference : Sahih Muslim 1649fIn-book reference : Book 27, Hadith 15USC-MSA web (English) reference : Book 15, Hadith 4049