Introduction
To answer this question it is worth considering the views of two famous shi'ite scholars:
Sheikh Saduq
Sheikh Saduq's (ra) view is that the Qur'an is categorically the word of Allah and remains unchanged from the time of the Prophet (saw) to his day:
Regarding the Quran our belief is that it is the word of Allah, His revelation which has descended from Him, it is His words and His book, Batil can neither enter it from the front, nor from the back. It has been revealed by the All Knowing and All Wise Allah, its tales are true and its sayings are firm. It is not a collection of forged useless incidents. Allah is its Creator, its Revealer, its Protector, and he speaks from it too."
E'tiqadaat by Sheikh Saduq, page 92
Sheikh Saduq (ra) further says:
"Our belief is that the Qur'an which Allah revealed to His Prophet Muhammad is (the same as) the one between the two covers (daffatayn). 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 …whoever asserts that we claim that it is greater in length than that, is a liar."
E'tiqadaat Sheikh Saduq 93, Published Iran
Sheikh Saduq (ra) then presents the following arguments to prove his claim:
"The narrations that have been revealed about the blessings (Thawab) for the recitation of the entire Quran, the narrations about the permissibility of reciting two Surahs in a single Rakah of non-obligatory (Nafl) Prayers, and the ones about the impermissibility of reciting two Surahs in a single Rakah of Obligatory prayers certify our claim that the Quran is the same that the people have in their hands at present, and the narrations that the whole of the Quran shouldn't be recited in a single night, and narrations about not reciting the whole of the Quran in less than three days verify our claim."
1. Ei'tidaqaat Sheikh Saduq, page 93
2. Tafseer al-Safi, page 13
Sheikh Baqar Majlisi
Shaykh Baqar Majlisi's view in the seventeenth century was similar, reflective of Shi'ite views about the Quran being the same unchanged book from the time it was revealed:
"If someone advance his doubts over present Quran being the actual book of Allah (swt) when there exist many traditions according to which the Imams (as) recited verses of the Quran in a manner that are different from the existing Quran for example "Ye are the best of Imams, evolved for mankind" , "Thus We have appointed you a middle Imams" and "They ask you the windfalls" - reply of such notion will be the same as already cited i.e these traditions that are counted amongst the 'Akhbar Ahad' and when they are (measured) against Quran then their authenticity is unsure therefore we do not rely on such traditions and haven't abandoned whatever is found in the present Quran because we have been ordered to act upon it …"
Bihar al Anwar, Volume 89 page 75
The three verses mentioned in the text are 3:110 , 2:143 and 8:1.
At another place whilst explaining the verse 15:9, Shaykh Baqair Majlisi advanced his unequivocal belief in the authenticity of the Quran in the following manner:
" إنا نحن نزلنا الذكر " أي القرآن " وإنا له لحافظون " عن الزيادة والنقصان والتغيير والتحريف
"We have revealed the Reminder" means the Quran "and We will most surely be its guardian" from addition, loss, change and Tahreef.
Bihar al-Anwar, Volume 9 page 113
In his other book Mirat al-Uqool, Volume 2 page 273, Shaykh Baqar Majlisi stated:
ليعلم أن للقرآن حملة يحفظونه عن التحريف في كل زمان
"It must be known that in every era, there are people who protect Quran from Tahreef"
Conclusion
Shi'ites do not believe the Qur'an has been altered and adopt the same version as that of the Sunnis.
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