وَمَا كُنتَ تَتۡلُواْ مِن قَبۡلِهِۦ مِن كِتَٰبٖ وَلَا تَخُطُّهُۥ بِيَمِينِكَۖ إِذٗا لَّٱرۡتَابَ ٱلۡمُبۡطِلُونَ ٤٨
And you did not recite before it any scripture, nor did you inscribe one with your right hand. Then [i.e., otherwise] the falsifiers would have had [cause for] doubt.
Translation: Saheeh International (English)
Each word's meaning and sound
O Messenger! You never read any book before the Qur’ān, nor did you write anything with your hand because you are unlettered and cannot read or write. Had you been able to read and write, the ignorant people would have doubted your messengership and used it as an excuse to say you were relating from the previous Books.
Al-Mukhtasar in interpreting the Noble Quran