Sahih Bukhari 6618

حَدَّثَنَا عَلِيُّ بْنُ حَفْصٍ ، وَبِشْرُ بْنُ مُحَمَّدٍ ، قَالَا : أَخْبَرَنَا عَبْدُ اللَّهِ أَخْبَرَنَا مَعْمَرٌ ، عَنْ الزُّهْرِيِّ ، عَنْ سَالِمٍ ، عَنْ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا ، قَالَ : قَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لِابْنِ صَيَّادٍ : خَبَأْتُ لَكَ خَبِيئَا ، قَالَ : الدُّخُّ ؟ قَالَ : اخْسَأْ ، فَلَنْ تَعْدُوَ قَدْرَكَ ، قَالَ عُمَرُ : ائْذَنْ لِي فَأَضْرِبَ عُنُقَهُ ، قَالَ : دَعْهُ إِنْ يَكُنْ هُوَ فَلَا تُطِيقُهُ ، وَإِنْ لَمْ يَكُنْ هُوَ فَلَا خَيْرَ لَكَ فِي قَتْلِهِ .

Narrated 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.