Sahih Bukhari Volume 008, Book 077, Hadith Number 615.
Narated By Ibn 'Umar : The Prophet said to Ibn Saiyad, "I have kept for you a secret." Ibn Saiyad said, "Ad-Dukh." The Prophet said, "Keep quiet, for you cannot go beyond your limits (or you cannot exceed what has been foreordained for you)." On that, 'Umar said (to the Prophet), "Allow me to chop off his neck!" The Prophet said, "Leave him, for if he is he (i.e., Ad-Dajjal), then you will not be able to overcome him, and if he is not, then you gain no good by killing him."