Allah Almighty says: "And We did not teach him poetry, nor is it befitting for him." [Surah Ya-Sin: 69]
And He said: "So believe in Allah and His Messenger, the unlettered Prophet." [Surah al-A'raf: 158]
Some of the scholars of tafsir said: "The unlettered one is he who neither reads a book nor writes with his right hand."
The Shaykh (Imam al-Bayhaqi) (may Allah have mercy on him) said: "This is the view of Muqatil bin Sulayman and other scholars of tafsir."
Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said, concerning the saying of Allah Almighty: "And you did not recite before it any scripture, nor did you write it with your right hand." [Surah al-Ankabut: 48]: He (peace and blessings be upon him) neither read nor wrote.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13286