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
حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى التَّيْمِيُّ، حَدَّثَنَا الْمُعْتَمِرُ، عَنْ أَبِيهِ، حَدَّثَنَا أَبُو السَّلِيلِ، عَنْ زَهْدَمٍ، يُحَدِّثُهُ عَنْ أَبِي مُوسَى، قَالَ كُنَّا مُشَاةً فَأَتَيْنَا نَبِيَّ اللَّهِ صلى الله عليه وسلم نَسْتَحْمِلُهُ . بِنَحْوِ حَدِيثِ جَرِيرٍ .
Abu Musa reported: We walked on foot and came to Allah's Apostle (may peace he upon him) asking him to provide us with mounts. The rest of the hadith is the same.
Reference : Sahih Muslim 1649hIn-book reference : Book 27, Hadith 17USC-MSA web (English) reference : Book 15, Hadith 4051