Since the Noble Qur’an was not revealed all at once, on the contrary, different verses of it were being revealed as and when appropriate, therefore, it was not possible from the very beginning to write and preserve it in a book form. So, during the initial stage of Islam, major emphasis was laid on memory as a means of preserving the Noble Qur’an.
When wahee (revelation) used to come in the very beginning, the Prophet (sallallahu alayhi wasallam) would tend to repeat its words instantly so that they would be memorized well enough. Thereupon, Allah Almighty directed him through the verses of Surah Qiyamah that he need not repeat words in a hurry immediately as revelation came. Allah Almighty would himself endow him with a memory that he will be unable to forget the words of the wahee once its descent has been completed.
So it was that the moment the Qur’anic verses would come to him, they would be committed to his memory the next moment. Thus, the blessed chest of the Prophet (sallallahu alayhi wasallam), was the most protected vault of the Noble Qur’an, in which there was no chance of even some common mistake, editing or alteration.
Moreover, as a matter of additional precaution, he used to recite the Qur’an before angel Jibra’eel alayhis salaam every year during the month of Ramadhan; and the year he left this mortal world he completed a cumulative review of Qur’anic recitation twice with Jibra’eel alayhis salaam. (Saheeh Bukhaari with Fat’hul Baari)
Again, as it was, he would not restrict his teachings of the Sahabah (Companions) to just the meaning of the Noble Qur’an, but had them memorize its words as well. Then, the revered Companions were themselves so enamored with a desire to learn and remember the Noble Qur’an that everyone of them was anxious to get ahead of the other. There were women who claimed no mahr (dowry) from their husbands except that they would teach the Qur’an.
Hundreds of Companions, freeing themselves from all other concerns, had devoted their whole lives for this purpose. Not only did they memorize the Qur’an but also went on repeating it within the nightly prayers. ‘When someone migrated from Makkah Mukarramah and came to Madeenah Munawwarah’, says Ubaadah Ibne Saamit radhiyallahu anhu, ‘the Prophet (sallallahu alayhi wasallam) would entrust him to one of us Ansar so that he could teach Qur’an to the new comer.’ The mosque of the Prophet (sallallahu alayhi wasallam) was so filled with voices generated by learners and teachers of the Qur’an that the noble Prophet (sallallahu alayhi wasallam) had to ask them to lower their voices so that mistakes are not made. (Manaahilul ‘Irfaan)
So, within a fairly short time, there was on hand a large group of the noble Companions who had the Glorious Qur’an all committed flawlessly to their memory. Included in this group were, in addition to the four guided Khulafaa, persons like Talhah, Sa’d, lbne Mas’ood, Huzayfah Ibne Yamaan, Saalim Mawlaa Aboo Huzayfah, Aboo Hurayrah, Abdullah Ibne Umar, Abdullah Ibne Abbaas, Amr Ibne Aas, Mu’aawiyah, Abdullah Ibne Zubayr, Abdullah Ibne Saa’ib, Aa’ishah, Hafsah, Umme Salamah, (radhiyallahu anhum).
In short, memorization of the Qur’an was given more emphasis in early Islam as this was the only protected and trustworthy method given the conditions of that time. The reason is that the number of people who could read or write was very limited in those days. The means of publishing books, such as printing press, etc., were not there. Therefore, in that situation, if writing was taken to be sufficient, it would have neither been possible to spread the Qur’an on an extensive scale nor to protect it reliably. In its place, Allah Almighty had blessed the people of Arabia with a memory of such dimensions that thousands of poetic lines would normally rest in the memory of one person after another.
Common ordinary villagers would remember by heart their genealogies and those of their families and unbelievably enough – even those of their horses! Therefore, this power of memory was well utilized for the conservation and protection of the Noble Qur’an and it was through it that the verses and chapters of the Noble Qur’an reached all over in to the far corners of Arabia.