Musnad Ahmad 5:114

Abu Sa'id narrated to us, Isra'il narrated to us, Abu Ishaq narrated to us, from al-Harith, from 'Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Hour will not be established until a man from my companions is sought, just as a lost animal is sought or searched for, but he will not be found."