Sahih Muslim 1649
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
Abu Musa reported:
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 .
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.