﴿ 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 you did not use to recite before this namely before the Qur’ān any other Book nor did you write it with your right hand for had you been reciting something and writing then those who follow falsehood the Jews would have had doubts about you and would have said ‘What the Torah states is that he will be unlettered ummī unable to read or write’.