﴿ وَمَا كُنتَ تَتْلُوا۟ مِن قَبْلِهِۦ مِن كِتَٰبٍۢ وَلَا تَخُطُّهُۥ بِيَمِينِكَ ۖ إِذًۭا لَّٱرْتَابَ ٱلْمُبْطِلُونَ ﴾

“for, [O Muhammad,] thou hast never been able to recite any divine writ ere this one [was revealed], nor didst thou ever transcribe one with your own hand or else, they who try to disprove the truth [of thy revelation] might indeed have had cause to doubt [it].”

(And thou (O Muhammad) were not a reader of any scripture before it) before the Qur'an, (nor didst thou write it with thy right hand, for) if you could read or write (then might those have doubted, who follow falsehood) then the Jews, Christians and idolaters would have doubted, for it is mentioned in the Scriptures of the Jews and Christians that you are unlettered.