THE ISSUE OF TAHRÍF IN THE HOLY QUR'ÁN
Tahríf refers to tampering with the letters or words of the verses of the Holy Qur'án, 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 Imám `Alí (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'án itself denies that any verses have been changed because Alláh 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'án further says:
"Verily those who disbelieve in the Reminder (Qur’án) 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." (Há Mím Sajdah, 41:41,42)
From the above verses we see that the Holy Qur'án is divinely protected from any tampering.
The issue of Tahríf is based on the original collection and compilation of the verses of the Holy Qur'án, 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.
References
The Collection & Preservation of Qur’an (Al Bayán fi Tafsíril Qur’án), Ayatulláh Syed Abul Qásim al Khu’í.
The Holy Qurán, S V Mír Ahmad `Alí