Musnad Abu Ya'la al-Mawsili 115:311

Ubayd Allah narrated to us, Khalid narrated to us, Sa'id narrated to us, Qatadah narrated to us, from Anas, who said: There was a panic in Medina, so the Messenger of Allah (ﷺ) rode a horse belonging to Abu Talha that was sluggish. Then the Prophet of Allah (ﷺ) returned and said: "We found it to be a sea among the seas." He said: "And it could not be outrun."