It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) had a horse that would run very swiftly with him, and he (peace and blessings be upon him) was greatly pleased with it, so the Messenger of Allah (peace and blessings be upon him) said: "This horse of mine is indeed a sea (as swift as the sea)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10357