The Messenger of Allah ﷺ passed by Ibn Sayyad along with a group of his companions, among them Umar bin Khattab. Ibn Sayyad was a young boy playing with other children near the fortress of Banu Maghalah, and he did not notice the Messenger of Allah ﷺ until he struck him on the back with his hand and said, "Do you bear witness that I am the Messenger of Allah?"Ibn Sayyad looked at him and said, "I bear witness that you are the Messenger of the unlettered ones."Then Ibn Sayyad said to the Prophet ﷺ, "Do you bear witness that I am the Messenger of Allah?"The Prophet ﷺ replied, "I believe in Allah and His messengers."Then the Prophet ﷺ asked, "What comes to you?"Ibn Sayyad said, "A truthful one and a liar come to me."The Prophet ﷺ said, "The matter has been confused for you."Then the Messenger of Allah ﷺ said, "I have concealed something for you," having hidden in his mind "the Day when the sky will bring forth a visible smoke" [Dukhan 44:10].Ibn Sayyad said, "It is ad-dukh."The Messenger of Allah ﷺ said, "Be gone, you will never exceed your measure."Umar said, "Messenger of Allah, allow me to strike off his head."The Messenger of Allah ﷺ replied, "If he is the one, you will not be given power over him, and if he is not, there is no good for you in killing him."Abdul-Razzaq said, meaning the Dajjal.
حدثنا عبد بن حميد أخبرنا عبد الرزاق أخبرنا معمر عن الزهري عن سالم
أن رسول الله ﷺ مر بابن صياد في نفر من أصحابه فيهم عمر بن الخطاب وهو يلعب مع الغلمان عند أطم بني مغالة وهو غلام فلم يشعر حتى ضرب رسول الله ﷺ ظهره بيده ثم قال أتشهد أني رسول الله فنظر إليه ابن صياد قال أشهد أنك رسول الأميين ثم قال ابن صياد للنبي ﷺ أتشهد أنت أني رسول الله فقال النبي ﷺ آمنت بالله وبرسله ثم قال النبي ﷺ ما يأتيك قال ابن صياد يأتيني صادق وكاذب فقال النبي ﷺ خلط عليك الأمر ثم قال رسول الله ﷺ إني خبأت لك خبيئا وخبأ له {يوم تأتي السماء بدخان مبين} فقال ابن صياد هو الدخ فقال رسول الله ﷺ اخسأ فلن تعدو قدرك قال عمر يا رسول الله ائذن لي فأضرب عنقه فقال رسول الله ﷺ إن يك حقا فلن تسلط عليه وإن لا يكنه فلا خير لك في قتله قال عبد الرزاق يعني الدجال
Sahih, agreed upon (by Bukhari and Muslim). Bukhari narrated it in Jihad and Expeditions, Chapter, How is Islam presented to a child, hadith no. 3055, from the narration of Mamar. Muslim narrated it, hadith no. 2930, from the narration of Zuhri, in a longer form.