Sahih Bukhari

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