I asked Ubayy bin Kaab, saying, "Abu al-Mundhir, your brother Ibn Masud says such and such."Ubayy replied, "I asked the Messenger of Allah ﷺ about it, and he said to me, 'It was said to me, so I said.' So we say as the Messenger of Allah ﷺ said."
سألت أبي بن كعب قلت: يا أبا المنذر، إن أخاك ابن مسعود يقول كذا وكذا، فقال أبي: سألت رسول الله ﷺ فقال لي: قيل لي فقلت قال: فنحن نقول كما قال رسول الله ﷺ.
Bukhari related it in the Virtues of the Quran (4977), from Ali bin Abdullah, who said, Sufyan told us, Abda bin Abu Lubaba told us, from Zirr bin Hubaysh, and he mentioned it.I say, this is because it is well known among many reciters and jurists that Ibn Masud did not write the two protective surahs (al-Muawwidhatayn) in his copy of the Quran. Rather, he would erase the two protective surahs from the copy of the Quran, saying, "The Messenger of Allah ﷺ was only commanded to seek refuge by means of them."But what is correct is what the majority of the companions held, those who memorized them and wrote them into the Imam's copies of the Quran, and had them dispatched to all regions. As for Abdullah bin Masud, perhaps he did not hear from the Prophet ﷺ that they were part of the Quran, or he forgot that, for it has been established that the Prophet ﷺ recited them in prayer.