Surah Al-'Ankabut — Ayah 48 (29:48)
وَمَا كُنتَ تَتۡلُواْ مِن قَبۡلِهِۦ مِن كِتَٰبٖ وَلَا تَخُطُّهُۥ بِيَمِينِكَۖ إِذٗا لَّٱرۡتَابَ ٱلۡمُبۡطِلُونَ ٤٨
English translation (Saheeh International)
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.
Brief commentary
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.