Musnad Ahmad 30:2420

Yahya narrated to us, from ‘Awf, who said: Khallas narrated to us, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "Before the Hour, there will be nearly thirty lying impostors (Dajjals). Each one of them will say: 'I am a prophet, I am a prophet.'"