Seyed Ali Shobayri, Seyed Ali Shobayri is of mixed Iranian and Scottish descent who found the path of the Ahlul Bayt (a) by his own research. He holds a BA in Islamic Studies from Middlesex University through the... Answer updated 1 year ago

Bismillah, 

Asalamu Alaykom, 

This is a belief which is held and originates from our brothers of the self proclaimed Sunni sect. Unfortunately this type of thought also found its way among some Shias. It has been answered already on different some Shia websites so I’ll transfer the narrations of our 9th Imam (as) for your benefit to this Q & A: 

Al-Saffar (May Allah be pleased with him) reported: Narrated Ali bin Asba`t or [and] others who had said: "I said to Abu` Jafar (al-Jawad): 'People claim that the Prophet (peace be upon him and his pure family) could neither read nor write!' So, he, peace be upon him, denied that and said: 'They have lied, May the wrath of Allah be upon them! How would that be? When Allah, the Exalted, has said: '(It is) He who raised among the Ummīyīna (i.e. maternals) a Messenger from amongst them, reciting upon them His signs and purifying them and teaching them the Book and the wisdom; even though beforehand they were in a clear straying' How did he teach them the Book and the Wisdom while he could neither read nor write?’ 

Then Ali bin Asba`t asked the Imam: "Why was he called al-Nabi al-Ummi? To which the Imam (peace be upon him) replied: "Because he was ascribed to Mecca. That is according to the words of Allah, the Most Exalted: 'That you may warn Umm al-Qura` (i.e. the mother of villages) and those around it.' Umm al-Qura` means Mecca. So, he was called Ummi.” (Basair al-darajat, By al-Saffar, vol. 5, p. 246). 

Furthermore, al-Saduq (May Allah be pleased with him) reported: Narrated Ja’far ibn Muhammad al-Sufi: “I asked Abu Ja’far Muhammad son of Ali al-Rida (peace be upon them both) saying: ‘O son of Allah’s Apostle, why was the Prophet called Ummi?’ He answered: ‘What do the people say?’ I said: ‘They claim that he was called Ummi because he was illiterate. 

“He replied: ‘They lie! May the curse of Allah be upon them, Allah has clearly said in His Book: ‘(It is) He who raised among the Ummīyīna (i.e. maternals) a Messenger from amongst them, reciting upon them His signs and purifying them and teaching them the Book and the wisdom’ How would he teach what he himself could not do? 

“By Allah, Allah’s Apostle (peace be upon him and his pure family) used to read and write in 72, or he said 73, languages. He was called Ummi because he was from Mecca. Mecca is one of the mother towns, and this is why Allah the Most Exalted has said: ‘So that you (Muhammad) may warn the mother of villages (i.e. Mecca) and whoever is around it’.” (Ma’ani al-Akhbar, By al-Saduq, p. 53). 

May Allah grant you success