Shia scholar allamah Ali Khan al-Madani ash-Shirazi in his book “Riyadu Salikin” (5/329) said:

وقال بعض المتأخرين من أصحابنا : إسقاط بعض القرآن وتحريفه ثبت من طرقنا بالتواتر معنى كما يظهر في كتب الأحاديث من أولها إلى اخرها ودلت الأخبار على وجود مصحف غير هذا المشهور بين الناس وهو موجود عند أهله والظاهر إنا مأمورين بقراءة ما في هذا القران ولا يجوز لنا الزيادة على ما فيه بما ورد في بعض الروايات أنه اسقط منه(2).إنتهى.

Translation:

“And some of our companions from later generation said: Omission of some parts of Quran and tampering (of other parts) established from our (i.m shia) way in mutawatir al-maana form. Like this is apparent to those who will investigate books of reports from the first one till the last. And it is evident from the reports that there is other copy (of Quran) not the one which is famous between people. It is present with its people (i.m ahlalbayt). It is apparent that we been ordered to read what is in this Quran (which is among us now). And it is not permitted for us to add to it anything, what is omitted from it in accordance to some reports”.