According to the Quran, the most abominable act in the sight of God is for someone to say what they do not do.
[61:2] O you who believe, why do you say what you do not do?
[61:3] Most abominable in the sight of GOD is that you say what you do not do.
(٦١:٢) يَاأَيُّهَا الَّذِينَ آمَنُوا لِمَ تَقُولُونَ مَا لَا تَفْعَلُونَ
(٦١:٣) كَبُرَ مَقْتًا عِنْدَ اللَّهِ أَنْ تَقُولُوا مَا لَا تَفْعَلُونَ
Yet, the upholders of Hadith besides the Quran claim that the prophet was illiterate, but that he decreed for others the utmost importance to know how to read and write.
According to Hadith, after the prophet arrived in Madina, he set up the Suffa, a school dedicated to instructing its attendees in the skills of literacy. The school even provided its pupils with food and shelter so they could maximize their time for their studies. It is estimated that some four-hundred to nine-hundred companions joined the Suffa. It is also told that Muhammad was one of the teachers, but apparently only taught the recitation of the Quran and not actual literacy to the students because as per traditionalist belief Muhammad was illiterate.*
This is most hard to believe, that the prophet would have put such a high emphasis on others to learn to read and write, yet never would have learned how to read or write himself. Not only does this concept of the prophet being illiterate contradict the Quran, as shown in previous articles, but such an understanding would have been a major sin if it was true. This is because it would mean that the prophet would have been committing one of the most abominable acts according to the Quran, that of saying what one does not do. If the prophet believed that literacy is of the utmost importance and encouraged hundreds of pupils to learn to read and write, it doesn’t make any sense to think that he would have stayed illiterate his whole life and not taken heed to his own advice.
*Source: Al-Kattani, at-Taratib al-Idariya, i:476-80. According to Qatada (61-117 A.H.) the number of pupils reached nine-hundred, while other scholars mention four-hundred.