In the book "Fadhael al-Quran" written by Abū ʻUbayd al-Qāsim ibn Sallām (a scholar from the Salaf), Ibn Umar says:
Verily among you people one would say that he has found the Quran whilst he is unaware of what the total quantity of the Quran was, because most of the Quran has been lost rather one should say that verily he has found the Quran that has appeared.’
~ Fadhael al-Quran - Volume 2, Page 135
I know, this book is not that much popular but the authenticity in Islam is not measured by book's popularity but Isnad & the Isnad of the above quote is 100% authentic as the chain of narration is:
Ibn Umar -> Nafi -> Ayub -> Ismael bin Ibrahim.
All these narrators are reliable according to Ibn Hajar and al-Dhahabi.
- See Al-Kashif, vol. 1 page 242 by Dhahabi and Taqrib al-Tahdib, vol. 1 page 90 by Ibn Hajar for Ismael Bin Ibrahim's reliability.
- See Siyar alam alnubala, vol. 6 page 15 by Dhahabi and Taqrib al-Tahdib, vol 1 page 116 by Ibn Hajar for Ayub's reliablity
- See Al-Kashif, vol. 2 page 315 by Dhahabi and Taqrib al-Tahdib, vol. 2 page 239 by Ibn Hajar for Nafi.
How do we explain the saying of Ibn Umar?