Sahih Muslim 1651
Sahih Muslim · Book 27: The Book of Oaths · 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
Tamim b. Tarafa reported:
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 .
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."