جامع الترمذي

It was narrated from Ibn 'Umar, that the Messenger of Allah(s.a.w) passed by Ibn Sayyad with a group of his Companions – among them `Umar bin Al-Khattab – while he was playing with two boys at the fort of Banu Maghalah, and he was a boy. He did not realize until the Messenger of Allah(s.a.w) struck him with his hand on his back, then he said: “Do you testify that I am the Messenger of Allah?” So Ibn Sayyad looked at him, and said: 'I testify that you are the Messenger to the illiterates.'” He said: “Then Ibn Sayyad said to the Prophet(s.a.w): 'Do you testify that I am the Messenger of Allah?' So the Prophet(s.a.w) said: 'I believe in Allah and His Messengers.' Then the Prophet(s.a.w) said: 'Who has come to you?' Ibn Sayyad said: 'A truthful one and a liar came to me.' So the Prophet(s.a.w) said: 'The matter has been confused for you.' Then the Messenger of Allah(s.a.w) said: 'I have concealed something from you.' And he had concealed (the verse): The day when the sky will bring forth a visible smoke. Ibn Sayyad said: 'It is, “Ad-Dukh.'” So the Messenger of Allah(s.a.w) said: 'Beat it! You can never surpass your ability.' `Umar said: 'O Messenger of Allah! Permit me to chop off his head!' The Messenger of Allah(s.a.w) said: 'If he is indeed him, then you will never overpower him, and if he is not, then there is no good in you killing him.'” (Sahih)