Musnad Abu Ya'la al-Mawsili 243:38

On the authority of Abu Musa, he said: Some people from my tribe came to me and said, "Ask the Messenger of Allah (peace and blessings of Allah be upon him) to provide us with mounts." He was providing mounts on camels. He said: So we came to the Prophet (peace and blessings of Allah be upon him) and asked him for mounts. He said, "By Allah, I will not provide you with mounts." So we left, and that weighed heavily upon us. He said: We remained for some days, and then camels were brought to him. He said, "Take these two pairs, take these two pairs," three times, [totaling] six mounts. I said to my companions, "He swore that he would not provide us with mounts, and I do not think but that he has forgotten. If we take them, we will not be blessed in them." So we said, "O Messenger of Allah, you had sworn three times that you would not provide us with mounts." He said, "And I am swearing now: By Allah, I will certainly provide you with mounts." He swore three times, then said, "Whoever swears an oath and then sees that which is better than it, let him do that which is better and offer expiation for his oath."