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
حَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمٍ، حَدَّثَنَا بَهْزٌ، حَدَّثَنَا شُعْبَةُ، حَدَّثَنَا سِمَاكُ بْنُ حَرْبٍ، قَالَ سَمِعْتُ تَمِيمَ بْنَ طَرَفَةَ، قَالَ سَمِعْتُ عَدِيَّ بْنَ حَاتِمٍ، أَنَّ رَجُلاً، سَأَلَهُ فَذَكَرَ مِثْلَهُ وَزَادَ وَلَكَ أَرْبَعُمِائَةٍ فِي عَطَائِي .
Tamim b. Tarafa reported: I heard 'Adi b. Hatim say that a person asked that and then narrated (the hadith) like one (mentioned above), but he made this addition:" Here are four hundred (dirhams) for you out of my gift."
Reference : Sahih Muslim 1651fIn-book reference : Book 27, Hadith 27USC-MSA web (English) reference : Book 15, Hadith 4061