Sahih Muslim 2924 a
'Abdullah reported:
We were along with Allah's Messenger (ﷺ) that we happened to pass by children amongst whom there was Ibn Sayyad. The children made their way but Ibn Sayyad kept sitting there (and it seemed) as if Allah's Messenger (ﷺ) did not like it (his sitting with the children) and said to him: May your nose he besmeared with dust, don't you bear testimony to the fact that I am the Messenger of Allah? Thereupon he said: No, but you should bear testimony that I am the messenger of Allah. Thereupon 'Umar b. Khattab said: Allah's Messenger, permit me that I should kill him. Thereupon Allah's Messenger (ﷺ) said: If he is that person who is in your mind (Dajjal ), you will not be able to kill him.
حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ، وَإِسْحَاقُ بْنُ إِبْرَاهِيمَ، - وَاللَّفْظُ لِعُثْمَانَ - قَالَ إِسْحَاقُ
 أَخْبَرَنَا وَقَالَ، عُثْمَانُ حَدَّثَنَا جَرِيرٌ، عَنِ الأَعْمَشِ، عَنْ أَبِي وَائِلٍ، عَنْ عَبْدِ اللَّهِ، قَالَ كُنَّا مَعَ
 رَسُولِ اللَّهِ صلى الله عليه وسلم فَمَرَرْنَا بِصِبْيَانٍ فِيهِمُ ابْنُ صَيَّادٍ فَفَرَّ الصِّبْيَانُ وَجَلَسَ
 ابْنُ صَيَّادٍ فَكَأَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَرِهَ ذَلِكَ فَقَالَ لَهُ النَّبِيُّ صلى الله عليه
 وسلم ‏"‏ تَرِبَتْ يَدَاكَ أَتَشْهَدُ أَنِّي رَسُولُ اللَّهِ ‏"‏ ‏.‏ فَقَالَ لاَ ‏.‏ بَلْ تَشْهَدُ أَنِّي رَسُولُ اللَّهِ ‏.‏ فَقَالَ
 عُمَرُ بْنُ الْخَطَّابِ ذَرْنِي يَا رَسُولَ اللَّهِ حَتَّى أَقْتُلَهُ ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم
 ‏"‏ إِنْ يَكُنِ الَّذِي تَرَى فَلَنْ تَسْتَطِيعَ قَتْلَهُ ‏"‏ ‏.‏