Sahih Bukhari | Volume 8 | Hadith Number 671
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."