Sahih Bukhari: Hadith 2937

Umar and a group of the companions of the Prophet (ﷺ) set out with the Prophet to Ibn Saiyad. He found him playing with some boys near the hillocks of Bani Maghala. Ibn Saiyad at that time was nearing his puberty. He did not notice (the Prophet's presence) till the Prophet (ﷺ) stroked him on the back with his hand and said, "Ibn Saiyad! Do you testify that I am Allah's Messenger (ﷺ)?" Ibn Saiyad looked at him and said, "I testify that you are the Apostle of the illiterates."

Then Ibn Saiyad asked the Prophet. "Do you testify that I am the apostle of Allah?" The Prophet (ﷺ) said to him, "I believe in Allah and His Apostles." Then the Prophet (ﷺ) said (to Ibn Saiyad). "What do you see?" Ibn Saiyad replied, "True people and false ones visit me." The Prophet said, "Your mind is confused as to this matter." The Prophet (ﷺ) added, " I have kept something (in my mind) for you." Ibn Saiyad said, "It is Ad-Dukh." The Prophet (ﷺ) said (to him), "Shame be on you! You cannot cross your limits." On that 'Umar said, "O Allah's Messenger (ﷺ)! Allow me to chop his head off." The Prophet (ﷺ) said, "If he should be him (i.e. Ad-Dajjal) then you cannot overpower him, and should he not be him, then you are not going to benefit by murdering him."