THE ISSUE OF TAHRIF IN THE HOLY QUR'AN

Tahrif refers to tampering with the letters or words of the verses of the Holy Qur'an, changing them from the original revealed form. There are many weak Hadith about such changes. Indeed, the Shi'ah are often accused of believing in a different Qur’an than the one in common use; one that contains many more verses, including some that clearly state the merits of Imam Ali (A). These accusations are baseless, as we do not and have never had a Qur’an any different from the one all Muslims use today.

The Holy Qur'an itself denies that any verses have been changed because Allah Himself pledges to guard it:

"Verily, We have sent down the Reminder (Qur’an) and We will most surely be its guardian." (al Hijr, 15:9)

The Holy Qur'an further says:

"Verily those who disbelieve in the Reminder (Qur’an) when it comes to them (should know) that it most surely it is a mighty Book. Falsehood shall never come to it from before it or from behind it, a revelation from the Wise, the Praised One." (Ha Mim Sajdah, 41:41,42)

From the above verses we see that the Holy Qur'an is divinely protected from any tampering.

The issue of Tahrif is based on the original collection and compilation of the verses of the Holy Qur'an, because there is no question that after the time of the third Caliph, the verses have been transmitted from Muslim to Muslim with any change. If there was any tampering, it would have occurred just after the passing away of the Holy Prophet (S).

From Hadith we know that this cannot have happened. Here are three proofs;

The Hadith of Thaqalayn is famous and undisputed. The Holy Prophet (S) said, "I am leaving behind "Thaqalayn" (the two weighty things). The Book of Allah (Qur’an) and my Ahlul Bayt…." The fact that he said he was leaving the Qur’an meant that it was already in a compiled form, and was known to the Muslims.
The Holy Prophet (S) said, "Jibrail had placed before me the Qur’an for review once a year and in the last year of my ministry, he did so twice which indicated that the time of my departure from this world was close at hand." This shows that both Alláh and the Holy Prophet (S) acted to ensure that no alteration in the verses could occur.
Various Ahadith from the Holy Prophet (S) describe reciting particular Suras in certain prayers. If these Suras were not known to the people, they would not have understood what to do. It therefore follows that the Holy Qur'an was available already compiled in the lifetime of the Holy Prophet (S).

References:
The Collection & Preservation of Qur’an (Al Bayan fi Tafsiril Qur’an),
By: Ayatullah Syed Abul Qasim al Khu’i.
The Holy Quran, S. V. Mir Ahmad Ali