Musnad Ahmad 32:892

Yahya narrated to us, from Humayd, from Anas, that Abu Musa asked the Prophet, peace and blessings of Allah be upon him, for a mount. He found him busy, so he said, "By Allah, I will not provide you with a mount." When he turned away, he called him and said, "You swore that you would not provide us with a mount." He replied, "And I swear that I will indeed provide you with a mount," and he provided them with a mount.