Ibn Hajar...
Ibn Hajar, in Sharh al-Bukhari says: Not writing these surahs by Ibn Mas'ud was known by all, so Ahmad and Ibn Hibban reported about him that he would never write the Mu’awwidhatan in his mushaf. And after citing all the narrations indicating that Ibn Mas'ud used to erase the Mu’awwidhatan from the copies of his mushaf, Ibn Hajar said: So, the claim of those saying it was falsified against him is rejected, and vilifying the sahih traditions without presenting any document can never be accepted!!
In Mushkil al-Qur’an Ibn Qutaybah writes: Ibn Mas'ud used to conjecture that the Mu’awwidhatan were never part of the Qur'an, since he saw the Prophet pronouncing them to seek God's protection for al-Hasan and al-Husayn, the fact upon which he based his conjecture. We never say that he was right in this and mistaken were the Muhajirun and Ansar. In regard of dropping them from his mushaf, it was not out of his surmise that they never belong to the Qur'an, I seek God's protection!
but he was of the opinion that the writing and collecting of the Qur'an between the two covers, had been because of fearing from suspicion and oblivion, and addition and omission, thinking this to be found in Surat al-Hamd, due to its shortness and necessity of its being learnt by all.
{dir="rtl"} To this a reference was made by al-Qadi in al-Intisar, when he said: The words of qunut reported to be recorded by Ubayy ibn Ka'b in his mushaf, could never be proved to be revealed Qur'anic verses but they were only some mode of supplication... as had they been of the Qur'an they would have been reported in the same way as a Qur'an whose veracity was established for all.
Also they might have contained some of the Qur'anic words that were abrogated afterwards but permitted to be used as supplication with being mixed then with non-Qur'anic words. All this was never confirmed to be narrated by him, but it was reported to be recorded in his mushaf. He is known to have recorded in his mushaf other than the Qur'an such as supplication and interpretation (ta’wil).