It is narrated from Sayyiduna Hudhayfah bin Asid (may Allah be pleased with him), (and I think he attributed it to the Messenger of Allah (peace and blessings be upon him)), that he said: "The Beast (Dabbat al-Ard) will emerge from the most sacred of mosques. The people will be sitting when suddenly the earth will rumble; and while they are in that state, the earth will suddenly split open."
Sufyan bin Uyaynah (may Allah have mercy on him) said: "This Beast will emerge when the imam sets off from Muzdalifah, and the one who goes ahead of the pilgrims (the herald) has been appointed for this very reason, to inform the people that the Beast has not yet emerged."
Imam al-Tabarani (may Allah have mercy on him) said: "No one narrated this hadith from Ibn Jurayj except Sufyan (may Allah have mercy on him), and Hamzah bin Sa'id (may Allah have mercy on him) is alone in narrating it from him."