What he said instead of arguing
The boy had answered his question with the same question.
آمَنْتُ بِاللَّهِ وَمَلاَئِكَتِهِ وَكُتُبِهِ
Āmantu bi-Llāhi wa malāʾikatihi wa kutubih
“I have believed in Allah, and His angels, and His books.”
Where this comes from
He met Ibn Sayyad on a road in Madinah with Abu Bakr and Umar, and asked him whether he testified that he was the Messenger of Allah. The boy threw the question back word for word. He did not repeat himself and he did not raise his voice. He stated his own position and then moved on to a different question.
The full narration
Abu Sa'id reported that Allah's Messenger (ﷺ) met him (Ibn Sayyad) and so did Abu Bakr and 'Umar on some of the roads of Medina. Allah's Messenger (ﷺ) said:Do you bear testimony to the fact that I am the Messenger of Allah? Thereupon he said: Do you bear testimony to the fact that I am the messenger of Allah? Thereupon Allah's Messenger (ﷺ) said: I affirm my faith in Allah and in His Angels and in His Books, and what do you see? He said: I see the throne over water. Whereupon Allah's Messenger (ﷺ) said: You see the throne of Iblis upon the water, and what else do you see? He said: I see two truthfuls and a liar or two liars and one truthful. Thereupon Allah's Messenger (ﷺ) said: Leave him He has been confounded
Source
Ṣaḥīḥ Muslim · 7346
- Agreed upon as authentic (al-Bukhārī / Muslim)