(14) باب يَحُولُ بَيْنَ الْمَرْءِ وَقَلْبِهِ
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: Sahih al-Bukhari 6618
In-book reference: Book 82, Hadith 24
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 77, Hadith 615