Musnad Ahmad 30:3734

'Ali narrated to us, Warqa' informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Hour will not be established until liars and impostors (Dajjals) are sent forth, numbering close to thirty, each of them claiming that he is the Messenger of Allah."