Musnad Ahmad 32:720

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Thabit, from Anas, who said: The people of Medina were alarmed one time, so the Prophet, peace and blessings of Allah be upon him, mounted a horse that was like a *muqraf* (a horse of mixed breed), and he rode it swiftly in their tracks. When he returned, he said: "We found it to be a sea."