Abu al-Darda (may Allah be pleased with him) said: Allah granted His Prophet (peace and blessings be upon him) camels as fay' (spoils). He (peace and blessings be upon him) distributed them, and Abu Musa (may Allah be pleased with him) asked him (peace and blessings be upon him) for camels. He (peace and blessings be upon him) refused, and this happened three times. He (peace and blessings be upon him) said: "No, by Allah! I will not give." He said: Four camels remained, and he (peace and blessings be upon him) said: "O Abu Musa (may Allah be pleased with him), take them." He said: O Prophet of Allah (peace and blessings be upon him)! I asked, and you (peace and blessings be upon him) swore that you would not give; could it be that you (peace and blessings be upon him) have forgotten? He said: "If I swear an oath about something, and then something else is better than it, I give the expiation for my oath and do the better thing."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19953