Sahih Bukhari Volume 008, Book 079, Hadith Number 712.
Narated By Zahdam al-Jarmi: We were sitting with Abu Musa Al-Ash'sari, and as there were ties of friendship and mutual favors between us and his tribe. His meal was presented before him and there was chicken meat in it. Among those who were present there was a man from Bani Taimillah having a red complexion as a non-Arab freed slave, and that man did not approach the meal. Abu Musa said to him, "Come along! I have seen Allah's Apostle eating of that (i.e., chicken)." The man said, "I have seen it (chickens) eating something I regarded as dirty, and so I have taken an oath that I shall not eat (its meat) chicken." Abu Musa said, "Come along! I will inform you about it (i.e., your oath).
Once we went to Allah's Apostle in company with a group of Ash'airiyin, asking him for mounts while he was distributing some camels from the camels of Zakat. (Aiyub said, "I think he said that the Prophet was in an angry mood at the time.") The Prophet said, 'By Allah! I will not give you mounts, and I have nothing to mount you on.' After we had left, some camels of booty were brought to Allah's Apostle and he said, "Where are those Ash'ariyin? Where are those Ash'ariyin?" So we went (to him) and he gave us five very fat good-looking camels. We mounted them and went away, and then I said to my companions, 'We went to Allah's Apostle to give us mounts, but he took an oath that he would not give us mounts, and then later on he sent for us and gave us mounts, perhaps Allah's Apostle forgot his oath. By Allah, we will never be successful, for we have taken advantage of the fact that Allah's Apostle forgot to fulfill his oath. So let us return to Allah's Apostle to remind him of his oath.' We returned and said, 'O Allah's Apostle! We came to you and asked you for mounts, but you took an oath that you would not give us mounts) but later on you gave us mounts, and we thought or considered that you have forgotten your oath.' The Prophet said, 'Depart, for Allah has given you Mounts. By Allah, Allah willing, if I take an oath and then later find another thing better than that, I do what is better, and make expiation for the oath.' "