It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever among you hears of the Dajjal, let him flee from him, for a man will come to him thinking that he is a believer, then he will follow him because of the doubtful things he sees."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15642