Musnad Ahmad 788:74

Sulayman ibn Harb narrated to us, Hammad ibn Zayd narrated to us, Ghaylan ibn Jarir narrated to me, from Abu Burdah ibn Abi Musa, from his father, who said: I came to the Messenger of Allah (peace and blessings of Allah be upon him) with a group of the Ash'ariyyin, asking him for mounts. He said: "By Allah, I will not provide you with mounts, and I have nothing upon which to mount you." We remained for as long as Allah willed, then he ordered for us three camels with white humps. When we set off, some of us said to others: "We came to the Messenger of Allah (peace and blessings of Allah be upon him) asking him for mounts, and he swore that he would not provide us with mounts. Let us return to him, so that we may remind him." He said: So we came to him and said: "O Messenger of Allah, we came to you asking for mounts, and you swore that you would not provide us with mounts, then you provided us with mounts." He said: "It is not I who provided you with mounts, but rather Allah, the Mighty and Majestic, provided you with mounts. By Allah, if Allah the Exalted wills, I do not swear an oath and then see something else as better than it, except that I come to that which is better, and I offer expiation for my oath," or he said: "except that I offer expiation for my oath, and come to that which is better."