It is not clearly known from which source finally the Quran was compiled whether from earlier fragmented writings preserved on Animal skins, bone, Date palm wood and animal shoulder or whether from people’s memory. I have seen some answers here which claims that sources are from both.
A written source is always undisputed and is logically the authentic source as compared to a memorised one. This is basically so because a memoriser can wilfully twist and recite (However trusted he may be) and there is no way of verifying it unlike a written source. With this I mean there is scope for N numbers of instances for omission and additions.
So if there were written sources of Quran then there was no need to consult the memorisers at the time of compilation of Quran. it is like taking best of the two sources. Was it therefore from both sources or either of the one source?