كتاب الأيمان
27  
The Book of Oaths
(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
(3)
باب نَدْبِ مَنْ حَلَفَ يَمِينًا فَرَأَى غَيْرَهَا خَيْرًا مِنْهَا أَنْ يَأْتِيَ الَّذِي هُوَ خَيْرٌ وَيُكَفِّرَ عَنْ يَمِينِهِ
Sahih Muslim 1649 f

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.
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 1649f
In-book reference : Book 27, Hadith 15
USC-MSA web (English) reference : Book 15, Hadith 4049
  (deprecated numbering scheme)