Musnad Ahmad 30:110

'Abd al-Rahman ibn Mahdi narrated to us, from Malik, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "The Hour will not be established until Dajjals (impostors) and liars are sent, numbering close to thirty, all of whom claim that he is the Messenger of Allah."