"And you did not recite before it"—meaning before the Quran—"any scripture, nor did you inscribe one with your right hand. Then [the followers of falsehood] would have suspected"—had you been able to read and write. And "the followers of falsehood" in the interpretation of some are those who did not believe among the People of the Book.
Muhammad said: The meaning according to this interpretation is that they find you described in their scriptures as unlettered, so if you had been able to write, they would have doubted.
Surat Al-Ankabut, from (verse 49) to (verse 52).
﴿وَمَا كُنْتَ تَتْلُو مِنْ قَبْلِهِ﴾ مِنْ قَبْلِ الْقُرْآنِ ﴿مِنْ كِتَابٍ وَلا تَخُطُّهُ بِيَمِينِكَ إِذًا لارْتَابَ المبطلون﴾ لَوْ كُنْتَ تَقْرَأُ وَتَكْتُبُ، وَ (المبطلون) فِي تَفْسِيرِ بَعْضِهِمْ: مَنْ لَمْ يُؤْمِنْ مِنْ أَهْلِ الْكِتَابِ.
قَالَ مُحَمَّدٌ: الْمَعْنَى عَلَى هَذَا التَّفْسِيرِ: أَيْ: إِنَّهُمْ يَجِدُونَكَ فِي كُتُبِهِمْ أُمِيًّا فَلَو كُنْتَ تَكْتُبُ لَارْتَابُوا.
سوررة العنكبوت من (آيَة ٤٩ - آيَة ٥٢).