Abu ‘Ubayd al-Qasim (d. 224 A.H) in his book “Fada’il al-Qur’an” said:

“We only saw that the reciters recited the Qira’at to those who have knowledge concerning it, and then they stuck firmly to what they knew of it out of fear that they would deviate from that which is between the two slates [i.e. the Qur’an] by either addition or subtraction.

Thus, they abandoned all Qira’at which opposed what was written (in the Mushaf). They paid no heed to the Arabic linguistic schools concerning them if they opposed the script of the Mushaf, even if the [recitation based on] an Arabic ruling was more clear than [what] the script [accommodated].

They [i.e. the reciters] considered the letters of the Mushaf and preserving them like the well-established Sunan (Prophetic ways) which no one is allowed to overstep.” (p: 361).

” وإنما نرى القراء عرضوا القراءة على أهل المعرفة بها ، ثم تمسكوا بما علموا منها؛ مخافة أن يَزيغوا عما بين اللوحين بزيادة أو نقصان .

ولذا تركوا سائر القراءات التي تخالف الكتاب، ولم يلتفتوا إلى مذاهب العربية فيها ، إذا خالف ذلك خط المصحف، وإن كانت العربية فيه أظهر بيانا من الخط .

ورأوا تتبع حروف المصاحف، وحفظها ؛ كالسنن القائمة التي لا يجوز لأحد أن يتعداها”. انتهى من “فضائل القرآن”. (ص: 361).