Abu Musa al-Ash'ari (may Allah be pleased with him) says: I came to the Prophet (peace and blessings be upon him) among a group of the Ash'aris so that he (peace and blessings be upon him) would provide us with mounts. He (peace and blessings be upon him) said: "By Allah! I will not give you mounts, and I do not have mounts on which to carry you." He says: We stayed as long as Allah willed. Then camels were brought to him (peace and blessings be upon him), and he (peace and blessings be upon him) ordered that we be given three camels with white humps. When we set off, we said, or some of us said: May Allah not bless us! We came to the Messenger of Allah (peace and blessings be upon him) to ask for mounts; he (peace and blessings be upon him) swore that he would not give us mounts, but later he provided us with mounts. They came to him (peace and blessings be upon him) and informed him, and he (peace and blessings be upon him) said: "It was not I who gave you mounts; rather, Allah gave you mounts. If I swear an oath and then see something better than it, I do that and make expiation for my oath."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19807