Zahdam al-Jarmi says: I came to Abu Musa (may Allah be pleased with him) while he was eating chicken. He said: Come close and eat. I said: I have sworn that I will not eat it. He said: Eat; I will soon tell you about this oath of yours. He says: So I came close and ate. He said: We came to the Prophet (peace and blessings be upon him) in a group of the Ash'aris, asking him for mounts, and he (peace and blessings be upon him) said: "By Allah! I will not give you mounts, nor do I have any." We stayed on, and then camels with high humps arrived, and he (peace and blessings be upon him) provided us with mounts. We had gone only a little way when we thought: We have made the Prophet (peace and blessings be upon him) forget his oath; by Allah! we will not prosper. So we came back to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) asked: "What has brought you back?" They said: O Prophet of Allah! You had sworn that you would not give us mounts; we feared that we would not be blessed in them, and that we may have made you forget your oath. He (peace and blessings be upon him) said: "I have not forgotten my oath; but whoever swears an oath and something else is better than it, let him do that and give the expiation for his oath."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19842