Musnad Ahmad 32:1527

Muhammad ibn Abdullah narrated to us, Humayd narrated to us, from Anas ibn Malik, who said: Abu Musa al-Ash'ari came to seek a mount from the Prophet, may God bless him and grant him peace, but he found him occupied. He said: "By God, I will not provide you with a mount." When he turned away, he called him and said: "O Messenger of God, I have sworn that you would not provide me with a mount." He said: "And I swear that I will provide you with a mount."