From Sahih Bukhari (Hadith 6618):
Narrated Ibn `Umar:The Prophet (ﷺ) 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 (ﷺ) ), "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
Reference: Bukhari 6618