Qur'anic Commentary: Its Advent and Development
Commentary on the words and expressions used in the Qur'an began at the time of the first revelation. The Prophet himself undertook the teaching of the Qur'an and the explanation of its meanings and intent.
Thus, in chapter XVI:44 God says, "And we have revealed to you the Remembrance that you may explain to mankind that which has been revealed for them." And He says in LXII :2, "He it is Who has sent among the unlettered ones a messenger of their own to recite to them His revelations and to make them grow and to teach them the Scriptures and wisdom."
At the time of the Prophet a group of men, on his orders, were instructed to read, record and learn the Qur'an by heart. When the Prophet's companions passed away, other Muslims took over the responsibility of learning and teaching the Qur'an; and so it has continued until the present day.