Musnad Abu Ya'la al-Mawsili 243:31

Khalaf ibn Hisham al-Bazzar narrated to us, Hammad ibn Zayd narrated to us, from Ghaylan ibn Jarir, from Abu Burdah, from Abu Musa, who said: I came to the Prophet, may Allah bless him and grant him peace, 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 I can mount you." He said: We remained for as long as Allah willed, then camels were brought to him, and he ordered three camels with white humps for us. He said: When we set off, we said—or some of us said to others—"May Allah not bless us. We came to the Messenger of Allah, may Allah bless him and grant him peace, asking him for mounts, and he swore that he would not provide us with mounts, then he provided us with mounts. Go to him and inform him." So he said, "It is not I who provided you with mounts, but rather Allah provided you with mounts. By Allah, if I will, I do not swear an oath and then see something else as better than it, except that I offer expiation for my oath and come to that which is better."