Musnad Ahmad 788:107

Isma'il narrated to us, Ayyub informed us, from al-Qasim al-Tamimi, from Zahdam al-Jarmi, who said: We were with Abu Musa, and chicken meat was brought for his meal. Among the people was a man from Banu Taym Allah, red-skinned as if he were a freed slave, and he did not draw near. Abu Musa said to him: "Draw near, for I have seen the Messenger of Allah (peace and blessings of Allah be upon him) eat from it." He said: "I saw him eat something that I found repulsive, so I swore that I would never eat it." He said: "Draw near, and I will tell you about that. I came to the Prophet (peace and blessings of Allah be upon him) with a group of the Ash'ariyyin to ask him for mounts, while he was distributing livestock from the charity livestock." Ayyub said: "I think he was angry." He said: "No, by Allah, I will not provide you with mounts, and I have nothing with which to provide you mounts." So we departed. Then, some camels from the spoils were brought to the Messenger of Allah (peace and blessings of Allah be upon him). He said: "Where are those Ash'ariyyin?" We were brought to him, and he ordered five camels with white humps for us. We set off, and I said to my companions: "We came to the Messenger of Allah (peace and blessings of Allah be upon him) to ask him for mounts, and he swore that he would not provide us with mounts, then he sent for us and provided us with mounts. I said: The Messenger of Allah (peace and blessings of Allah be upon him) has forgotten his oath. By Allah, if we let the Messenger of Allah (peace and blessings of Allah be upon him) be heedless of his oath, we will never succeed. Let us return to the Messenger of Allah (peace and blessings of Allah be upon him) and remind him of his oath." We returned to him and said: "O Messenger of Allah, we came to you to ask you for mounts, and you swore that you would not provide us with mounts, then you provided us with mounts, so we knew—or we thought—that you had forgotten your oath." The Prophet (peace and blessings of Allah be upon him) said: "Go, for it is Allah, the Mighty and Majestic, who provided you with mounts. By Allah, if I swear an oath and then see something else as better than it, I will do that which is better and break my oath."