Sahih Bukhari Volume 8, Book 83, Hadith 6682

حَدَّثَنَا أَبُو مَعْمَرٍ، حَدَّثَنَا عَبْدُ الْوَارِثِ، حَدَّثَنَا أَيُّوبُ، عَنِ الْقَاسِمِ، عَنْ زَهْدَمٍ، قَالَ كُنَّا عِنْدَ أَبِي مُوسَى الأَشْعَرِيِّ قَالَ أَتَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم فِي نَفَرٍ مِنَ الأَشْعَرِيِّينَ، فَوَافَقْتُهُ وَهْوَ غَضْبَانُ فَاسْتَحْمَلْنَاهُ، فَحَلَفَ أَنْ لاَ يَحْمِلَنَا ثُمَّ قَالَ ‏"‏ وَاللَّهِ إِنْ شَاءَ اللَّهُ لاَ أَحْلِفُ عَلَى يَمِينٍ فَأَرَى غَيْرَهَا خَيْرًا مِنْهَا، إِلاَّ أَتَيْتُ الَّذِي هُوَ خَيْرٌ وَتَحَلَّلْتُهَا ‏"‏‏

Narrated By Abu Musa Al-Ash'ari : I went along with some men from the Ash-ariyin to Allah's Apostle and it happened that I met him while he was in an angry mood. We asked him to provide us with mounts, but he swore that he would not give us any. Later on he said, "By Allah, Allah willing, if ever I take an oath (to do something) and later on I find something else better than the first, then I do the better one and give expiation for the dissolution of my oath."