Sahih Bukhari Volume 008, Book 078, Hadith Number 644.
Narated By Zahdam : There was a relation of love and brotherhood between this tribe of Jarm and Al-Ash'ariyin. Once we were with Abu Musa Al-Ash'ari, and then a meal containing chicken was brought to Abu Musa, and there was present, a man from the tribe of Taimillah who was of red complexion as if he were from non-Arab freed slaves. Abu Musa invited him to the meal. He said, "I have seen chickens eating dirty things, so I deemed it filthy and took an oath that I would never eat chicken." On that, Abu Musa said, "Get up, I will narrate to you about that. Once a group of the Ash'ariyin and I went to Allah's Apostle and asked him to provide us with mounts; he said, 'By Allah, I will never give you any mounts nor do I have anything to mount you on.' Then a few camels of war booty were brought to Allah's Apostle , and he asked about us, saying, 'Where are the Ash-'ariyin?' He then ordered five nice camels to be given to us, and when we had departed, we said, 'What have we done? Allah's Apostle had taken the oath not to give us any mounts, and that he had nothing to mount us on, and later he gave us that we might ride? Did we take advantage of the fact that Allah's Apostle had forgotten his oath? By Allah, we will never succeed.' So we went back to him and said to him, 'We came to you to give us mounts, and you took an oath that you would not give us any mounts and that you had nothing to mount us on.' On that he said, 'I did not provide you with mounts, but Allah did. By Allah, if I take an oath to do something, and then find something else better than it, I do that which is better and make expiation for the dissolution of the oath.'"