The entire Quran was revealed to the Prophet (PBUH) in his lifetime. The period of revelation was 23 years, up to just a few days before the Prophet (PBUH) passed away.  However, the Prophet realized the importance of the preservation of the words of Allah during his lifetime and took active measures to ensure that these divine words were preserved for his upcoming generations, which he called Ummah.

So the conservation of the Quranic verses began very soon. The Prophet (PBUH) did not know how to read and write but such was his dedication towards preserving the words that were revealed to him that he brought about a scribe system to record the ayahs. He had many literate companions and friends, the most notable among them was Zaid ibn Thabit, who took upon himself to safeguard the quranic verses.

Hence began the process of putting together the ayahs brought down upon the Holy Prophet (PBUH) by putting together a committee which accepted only those scripts that were written down in the presence of the Holy Prophet (PBUH) and those that could be accounted for by 2 witnesses who had heard the Prophet (PBUH) recite those verses. This practice led to the complete Quran being  in a written form in the lifetime of the Prophet. However, the task of collecting the Qur'an as a book was undertaken by Abu Bakr, the first successor to the Prophet.

Another way of keeping the words of the Quran safe was to memorize them. The Holy Prophet (PBUH) himself memorized the verses as soon as they were revealed to him.  When the followers of the Prophet (PBUH) came to him with his problems or asked for his suggestions to carry out certain duties in their lives in the light of Islam, he would quote the verses of the Quran to them to help them out. This, in itself, led to the holy Prophet (PBUH) strengthening his own hold on the words of the Quran.

The Holy Prophet (PBUH) also encouraged his companions to learn and memorize the Quran as well. In a tradition of the Holy Prophet he is known to have said,

” 'The most superior among you (Muslims) are those who learn the Qur'an and teach it'. “

Hence his thinking was really far fetched. He not only advocated the memorizing of the Quran but also teaching it to others so that they in turn would further teach it to others and the message of Islam would spread far and wide even after he passed away.

Thus, the Quran, due to the efforts of the companions of the Prophet and the Holy Prophet himself, became the only religious script that has retained its originality in over 15 centuries. Allah knew, when He was revealing the Quran that it would be preserved as proven by the ayah:

“Verily, We have revealed the Reminder, and verily We shall preserve it.” (Quran 15:9)

Hence the Quran, according to Allah’s promise, was preserved in both oral, written and DIVINE form!