We read in Tafseer al-Kabeer:
نقل في الكتب القديمة أن ابن مسعود كان ينكر كون سورة الفاتحة من القرآن وكان ينكر كون المعوذتين من القرآن
“In some of the previous books it is written that Ibn Masud would reject Surah Fatihah and Mu'awwidh-at [Mauzatain] as being a part of the Quran”
We appeal to justice amongst our readers, after reading the above cited references that clearly state that the Sunni Imams like Imam Abu Hanifa and Imam Malik and the Sahabi Ibn Masud clearly opposed Surah Fatihah and Mauzatain being a part of the Holy Quran.
lol at your appeal.
Lets quote what Razi said , without doing tahreef to what he said.
نقل في الكتب القديمة أن ابن مسعود كان ينكر كون سورة الفاتحة من القرآن وكان ينكر كون المعوذتين من القرآن ، واعلم أن هذا في غاية الصعوبة لأنا إن قلنا إن النقل المتواتر كان حاصلاً في عصر الصحابة بكون سورة الفاتحة من القرآن فحينئذ كان ابن مسعود عالماً بذلك فإنكاره يوجب الكفر أو نقصان العقل وإن قلنا إن النقل المتواتر في هذا المعنى ما كان حاصلاً في ذلك الزمان فهذا يقتضي أن يقال إن نقل القرآن ليس بمتواتر في الأصل وذلك يخرج القرآن عن كونه حجة يقينية والأغلب على الظن أن نقل هذا المذهب عن ابن مسعود نقل كاذب باطل وبه يحصل الخلاص عن هذه العقدة
تفسير الفخر الرازي 1/178
Some of the other references , the shia site give are
Tafseer Dur al Manthur
قال السيوطي :
( أخرج أحمد والبزار والطبراني وابن مردويه من طرق صحيحة عن ابن عباس وابن مسعود أنه كان يحك المعوذتين من المصحف
Suyuti said that it has been authentically narrated in Ahmad, Bazzar , Tabrani and Ibn Mardudiya , from Ibn Abbas and Ibn Masud that they considered Muazatain a part of the Mushaf (i.e Quran)
الدّر المنثور 8/683
Tafseer Iteqan
Iteqan was also written by Imam Suyuti and we already mentioned what he said in his other book, Durre Manthur regarding Ibn Masud
Also, it is written in Iteqan,
وقال ابن حزم : ( هذا كذب على ابن مسعود وموضوع وإنما صح عنه قراءة عاصم عن زر عنه وفيها المعوذتان والفاتحة )
Rough translation : Ibn Masud said that It is a lie upon Ibn Masud and a fabrication and the correct thing is that he did consider Muazatain and Fatiha in the Quran.
الإتقان في علوم القرآن للسيوطي 1/213 ، البرهان في علوم القرآن للزكشي 2/128
The correct thing is , that he did not wrote Muazatain and Fatiha in the Mushaf he wrote, because they are very short surahs (ch 1, 113, and 114) and almost every Muslim learns it by heart, so for this purpose, he did not wrote them in his Mushaf, [See Maqalat al Kauthari, Page 16, Tafseer Qurtabi, volume 1, Page 115 , 184] some people thought that he did not believe these to be a part of Quran, so they were mistaken due to this reason that he had not included these chapters in his mushaf, and the narration mentioned in Musnad Ahmad in which it is clearly narrated from Ibn Masud that "Muazatain are not a part of Quran" , first of all, it is only narrated from Abdur Rehman bin Yazeed, and it is against the mutawatir ahadith which confiirms that Abdullah ibn Masud did consider Fatiha and Muazatain a part of Quran, secondly, this hadith is "shaz" due to its being against the mutawatir ahadith
Shaz (شاذ)
Literal Meaning: Isolated.
Technical Meaning: Shaz Hadith is that Hadith wherein a comparatively less authentic narrator of Hadith opposes a more authentic narrator. This opposition may be either in Sanad (chain of transmission) or in Matan (text).
Status: This Hadith is rejected and not accepted.
Difference between Shaz and Munkar
The narrator of Shaz is authentic who narrates against a more authentic narrator whereas the narrator of Munkar is weak or less authentic.
http://publications.islamkashmir.org/classification-of-hadith
Again,
Shia scholar "Sayyad Muhammad Aun Naqwi" says in his book "Irfan ul Quran" , page 364-366, quoting Sayyad Murtaza Ali ibn Al Hussain Al Mooswi (d. 436 H)
"A group of sahaba were appointed to learn Quran by heart. And they would recite the holy Quran in front of the Prophet (peace be upon him) after learning it by heart. And especially a group of sahaba amongst whom Abdullah ibn Masud and Abi bin Kaab were also included, completed the recitation of Quran in front of the Prophet (peace be upon him) many many times."
Shia website al-islam.org says:
Another member of this group was Abdullah ibn Masood, a young Muslim. He was distinguished among the companions of the Prophet by his knowledge and learning, and he was one of the earliest huffaz (men who knew Al-Qur’an al-Majid by heart) in Islam. As each new verse was revealed, he heard it from the Prophet and memorized it.
http://www.al-islam.org/restatement/10.htm
So how is it possible that a person who recites the whole Quran in front of the Prophet (peace be upon him) many many times don't consider 3 chapters of the Quran a part of it? Its impossible.