Sahih Muslim Book 41, Hadith Number 6990.

‘Abdullah reported: We were along with Allah’s Messenger (may peace be upon him) that we happened to pass by children amongst whom there was Ibn Sayyad. The children made their way but Ibn Sayyad kept sitting there (and it seemed) as if Allah’s Messenger (may peace be upon him) did not like it (his sitting with the children) and said to him: May your nose he besmeared with dust, don’t you bear testimony to the fact that I am the Messenger of Allah? Thereupon he said: No, but you should bear testimony that I am the messenger of Allah. Thereupon ‘Umar b. Khattab said: Allah’s Messenger, permit me that I should kill him. Thereupon Allah’s Messenger (may peace be upon him) said: If he is that person who is in your mind (Dajjal), you will not be able to kill him.