The Unlettered Prophet
Question

                Is there any evidence that the Prophet Muhammad was unable to read or write?
Praise be to Allah.

Allaah says (interpretation of the meaning):
"Those who follow the Messenger, the Prophet who can neither read not write whom they find written with them in the Tawraat and the Injeel, - he commands them for al-Ma’roof (i.e., Islamic monotheism and all that Islaam has ordained); and forbids them from al-Munkar (i.e., disbelief, polytheism of all kinds, and all that Islaam has forbidden); he allows them as lawful al-Tayyibaat (i.e., all good and lawful as regards things, deeds, beliefs, persons, foods, etc.), and prohibits them as unlawful al-khabaa’ith (i.e., all evil and unlawful as regards things, deeds, beliefs, persons, foods, etc.), he releases them from their heavy burdens (of Allaah’s Covenant) and from the fetters (bindings) that were upon them. So those who believe in him (Muhammad), honor him, help him and follow the light (the Qur’aan) which has been sent down with him, it is they who will be successful."
[al-A’raaf 7:157]

Al-Qurtubi, may Allaah have mercy on him, said in his tafseer of this aayah: "Allaah says ‘al-ummi.’ Ibn ‘Abbaas, may Allaah be pleased with him, said: ‘Your Prophet was unlettered, unable to read or write or calculate.’ Allaah says (interpretation of the meaning): ‘Neither did you (O Muhammad) read any book before it (this Qur’an), nor did you write any book (whatsoever) with your right hand . . .’
[al-‘Ankaboot 29:48]."

Ibn Katheer, may Allaah have mercy on him, said in his tafseer of the second aayah quoted [al-‘Ankaboot 29:48]:
"Allaah says (interpretation of the meaning): ‘Neither did you (O Muhammad) read any book before it (this Qur’aan), nor did you write any book (whatsoever) with your right hand . . .’ i.e., ‘you (O Muhammad) lived among your people for a while before you brought this Qur’aan to them, and you never read any book or were able to write anything. Everyone among your people and others knows that you are an unlettered man, who does not read or write.’ This is how he was described in the previous Books as Allaah said (interpretation of the meaning): ‘Those who follow the Messenger, the Prophet who can neither read not write whom they find written with them in the Tawraat and the Injeel, - he commands them for al-Ma’roof (i.e., Islamic monotheism and all that Islaam has ordained); and forbids them from al-Munkar (i.e., disbelief, polytheism of all kinds, and all that Islaam has forbidden) . . .’ [al-A’raaf 7:157].

Hence the Prophet will remain unable to write even one line or one letter, until the Day of Resurrection. He had scribes who would write down in front of him the Revelation and letters to different regions. . . . Allaah says (interpretation of the meaning): ‘. . . In that case, indeed, the followers of falsehood might have doubted.’ [al-‘Ankaboot 29:48], i.e., if you had been good at it (reading and writing), some of the ignorant people would have doubted you and said that you had learnt this from the previous Books left by the Prophets. Indeed, they say this despite the fact that they know he was unlettered and unable to write, as it says in the Qur’aan (interpretation of the meaning): ‘And they say: "Tales of the ancients, which he has written down, and they are dictated to him morning and afternoon."’ [al-Furqaan 25:5]"

Allaah says (interpretation of the meaning):

"He it is Who sent among the unlettered ones a Messenger from among themselves, reciting to them His Verses, purifying them (from the filth of disbelief and polytheism), and teaching them the Book and al-Hikmah (al-Sunnah). And verily, they had been before in manifest error."
[al-Jumu’ah 62:2]

Al-Qurtubi, may Allaah have mercy on him, said in his tafseer of this aayah:
"It was said: ‘the unlettered ones’ means those who do not write. Such were the Quraysh. Mansoor reported from Ibraaheem, who said: ‘Al-ummi (the unlettered one) is the one who neither reads nor writes. "A Messenger from among themselves" means Muhammad , who was unlettered and never read a book nor learned how.’ Al-Mawardi said: ‘What is good about the fact that Allaah sent an unlettered Prophet? There are three things:
(i) his message fulfilled the foretelling of the previous Prophets;
(ii) this made him similar to and closer to other Prophets (illiterates);
(iii) this would eliminate all suspicion that he had learned the message he preached from books and writings that he had read."

I say: all of this is evidence of the miraculous nature and truth of his Prophethood.

(The above has been summarized from the Tafseer of al-Qurtubi, may Allaah have mercy on him).