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, ʿUmar 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."  

قَالَ النَّبِيُّ ﷺ 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  

We were walking with Messenger of Allah ﷺ that Ibn Sayyad happened to pass by him. Messenger of Allah ﷺ said to him: I have concealed for you (something to test you, so tell me that). He said: It is Dukh. Thereupon Messenger of Allah ﷺ said to him: Be off. You cannot get farther than your rank, whereupon 'Umar said: Messenger of Allah, permit me to strike his neck. Thereupon Messenger of Allah ﷺ said: Leave him; if he is that one (Dajjal) whom you apprehend, you will not be able to kill him.  

كُنَّا نَمْشِي مَعَ النَّبِيِّ ﷺ فَمَرَّ بِابْنِ صَيَّادٍ فَقَالَ لَهُ رَسُولُ اللَّهِ ﷺ قَدْ خَبَأْتُ لَكَ خَبِيئًا فَقَالَ دُخٌّ فَقَالَ رَسُولُ اللَّهِ ﷺ اخْسَأْ فَلَنْ تَعْدُوَ قَدْرَكَ فَقَالَ عُمَرُ يَا رَسُولَ اللَّهِ دَعْنِي فَأَضْرِبَ عُنُقَهُ فَقَالَ رَسُولُ اللَّهِ ﷺ دَعْهُ فَإِنْ يَكُنِ الَّذِي تَخَافُ لَنْ تَسْتَطِيعَ قَتْلَهُ  

We were walking with Messenger of Allah ﷺ that Ibn Sayyad happened to pass by him. Messenger of Allah ﷺ said to him: I have concealed for you (something to test you, so tell me that). He said: It is Dukh. Thereupon Messenger of Allah ﷺ said to him: Be off. You cannot get farther than your rank, whereupon 'Umar said: Messenger of Allah, permit me to strike his neck. Thereupon Messenger of Allah ﷺ said: Leave him; if he is that one (Dajjal) whom you apprehend, you will not be able to kill him. (Using translation from Muslim 2924b)   

كُنْتُ أَمْشِي مَعَ رَسُولِ اللَّهِ ﷺ فَمَرَّ بِابْنِ صَيَّادٍ فَقَالَ النَّبِيُّ ﷺ «إِنِّي قَدْ خَبَّأْتُ لَكَ خَبْأً» فَقَالَ هُوَ الدَّخُّ فَقَالَ النَّبِيُّ ﷺ «اخْسَأْ فَلَنْ تَعْدُوَ قَدْرَكَ» قَالَ فَقَالَ عُمَرُ ؓ دَعْنِي فَأَضْرِبَ عُنُقَهُ قَالَ «لَا إِنْ يَكُنِ الَّذِي تَخَافُ فَلَنْ تَسْتَطِيعَ قَتْلَهُ»