Musnad Ahmad 32:116

Ibn Abi ‘Adi narrated to us, from Humayd, from Anas, that Abu Musa asked the Prophet, may Allah bless him and grant him peace, for a mount. He found him occupied, so he said, "By Allah, I will not provide you with a mount." When he turned to leave, he called him and provided him with a mount. He said, "O Messenger of Allah, you swore that you would not provide me with a mount." He said, "I am swearing that I will provide you with a mount."