Majlisi has written a whole chapter titled “Abdullah ibn Masud was the most learned about Quran of all the sahaba ”
He writes in his book “Haq ul Yaqin”
The author of Istiab has recorded that the Prophet (s) said “Learn Quran from these four people and began from Abdullah ibn Masud and after him Muadh bin Jabal , Ubai bin Kaab, and slave of salim hudhaifa and said whoever wants to read Quran in better way, than he should read from the qirat of Ibn Umm Abd i.e according to the qirat of Ibn Masud and it has been narrated from Abu Wayel that he said that I heard Ibn Masud saying that ” I am the most knowledgable of the book of God in this ummah, and I am not superior to them, and no Surah in the book of God was revealed and no verse but I know about what it was revealed and when it was revealed and Abu Wayel says that no one rejected him. and it has been narrated from Abu Zubyan that he said that Ibn Abbas said to me that every year, Jabriel would present the Quran to Prophet (s) one time, and in the year in which the Prophet (s) died, he presented Quran twice. At that time, Ibn Masud was present with the Prophet (s). Hence, whatever changes happened in the Holy Quran and whatever was abrogated, he got its knowledge. People asked Ali about Ibn Masud, he said “He read Quran and understood the sunnah and this is enough for him”.
Haq ul Yaqin, Vol. 1, p. 304
Urdu Translation by Sayyid Basharat Hussain
Published by Majlis Ilmi Islami (Pakistan)