Sahih al-Bukhari 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."
حَدَّثَنَا عَلِيُّ بْنُ حَفْصٍ، وَبِشْرُ بْنُ مُحَمَّدٍ، قَالاَ أَخْبَرَنَا عَبْدُ اللَّهِ، أَخْبَرَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنْ سَالِمٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم لاِبْنِ صَيَّادٍ ‏"‏ خَبَأْتُ لَكَ خَبِيئًا ‏"‏‏.‏ قَالَ الدُّخُّ‏.‏ قَالَ ‏"‏ اخْسَأْ فَلَنْ تَعْدُوَ قَدْرَكَ ‏"‏‏.‏ قَالَ عُمَرُ ائْذَنْ لِي فَأَضْرِبَ عُنُقَهُ‏.‏ قَالَ ‏"‏ دَعْهُ، إِنْ يَكُنْ هُوَ فَلاَ تُطِيقُهُ، وَإِنْ لَمْ يَكُنْ هُوَ فَلاَ خَيْرَ لَكَ فِي قَتْلِهِ ‏"‏‏.‏