Sahih Muslim Book 41, Hadith Number 6991.

‘Abdullah reported: We were walking with Allah’s Messenger (may peace be upon him) that Ibn Sayyad happened to pass by him. Allah’s Messenger (may peace be upon him) said to him: I have concealed for you (something to test you, so tell me that). He said: It is Dukh. Thereupon Allah’s Messenger (may peace be upon him) said to him: Be off. You cannot get farther than your rank, whereupon ‘Umar said: Allah’s Messenger, permit me to strike his neck. Thereupon Allah’s Messenger (may peace be upon him) said: Leave him; if he is that one (Dajjal) whom you apprehend, you will not be able to kill him.