All grand scholars of the Imami Shia are in agreement that the Quran which is at present among the Muslims is the very same Quran that was sent down to the Holy Prophet, and that it has not been altered. Nothing has been added to it, and nothing is missing from it. The Quran which was compiled by Imam Ali (excluding the commentaries) and the Quran that is in the hand of people today, are identical in terms of words and sentences. No word, verse, chapter is missing. The only difference is that the current Quran (collected by the companions) is not in the order that was revealed.
The completeness of Quran is so indisputable among Shia that the great Shia
scholar, Abu Ja'far Muhammad Ibn Ali Ibn al-Husain Ibn Babwayh, known as
"Shaikh Saduq" (309/919-381/991), wrote:
"Our belief is that the Quran which Allah revealed to His Prophet
Muhammad is (the same as) the one between the two covers (daffatayn).
And it is the one which is in the hands of the people, and is not
greater in extent than that. The number of Surahs as generally
accepted is one hundred and fourteen...And he who asserts that we say
that it is greater in extent than that, is a liar."
Shi'i reference: Shi'ite Creed (al-I'tiqadat al-Imamiyyah), by Shaikh
Saduq, English version, p77.
It should be noted that Shaikh Saduq (RA) was the greatest scholars of
Hadith among the Imami Shia and was given the name of Shaikh al-Muhaddithin
(i.e., the most eminent of the scholars of Hadith). And since he wrote the
above in a book with the name of "The beliefs of the Imami Shia," it is
quite impossible that there could be any authentic Hadith in contrary to
it. It is noteworthy that Shaikh Saduq lived at the time of minor
occultation of Imam Mahdi (AS) and he is one of the earliest Shia scholars.
He had the honor that he was born with the prayer of Imam Mahdi (AS).
For a more detailed discussion of completeness of Quran as well as the
opinion of the Shia, interested readers may look at "al-Bayan," by Abul
Qasim al-Khoei, pp 214-278.
Some ignorant opponents of the Shia mentioned that we apply al-Taqiyya
(dissimulation) and we do not release our actual belief on Quran. These
people never tried to understand that Taqiyya is for the time when my life
or the life of the other fellow is in danger. There is no need to conceal
my belief here since I am not under prosecution. The above article is
witness to what I say. Taqiyya is not a good excuse for these people in
front of Allah to disregard what Shia present. They have liberty to check
the traditions which we have mentioned in different articles, or they can
else ask their "honest" scholars to do that.
Source:
http://www.al-islam.org/encyclopedia/chapter8/4.html