" Allah effaces and confirms what He wishes. With/By Him is the mother of the book"
The designation "Umm al Kitab" occurs three times in the Qur’an in Surat aal Imran 3:7, Surat al Raad13:39, Surat al Zukhruf 43:4
Imam Haqqi :
The Arabs call Umm (mother) anything that is a source from which other (things) flow continuously. For example, the brain is referred to as the mother of the head; Makka is referred to as mother of the cities (Umm al Qura)...This also means that it is its unchangeable origin.
Umm al Kitab is what Allah (swt) has written in the eternality before time/sempiternity (Azal); it is a knowledge originating from sempiternity (Azal) for eternity (Abad). Its existence is in Divine Perpetuity (Sarmadi), it is established in its own-essence/self (not contingent on any other knowledge), it encompasses everything with no addition or lack, everything in it is decreed in a specific measure.
Imam al-Razi adds to the above: There are 2 further sayings on this matter:
1. Umm al Katab is the Preseved Tablet (al Lawh al Mahfouz) wherein all the events of the higher and lower Universes are fixed as stated by the Prophet (saaws): “ Allah was and nothing was with Him, then He created the Tablet and fixed in it the various states of everything in the creation until the occurrence of the Hour.”
2. Umm al Kitab is the knowledge of Allah the most High, for He is all-Knowledgeable of all the knowledges of what is existent (Mawjood) and what is non-existent (Maa’doom). Even if the existent and the non-existent were to incur changes, the Knowledge of Allah of both does not change. That is what is referred to as Umm al-Kitab.
* There is an implicit and at times explicit understanding that Umm al Kitab is the origin of all the scriptures.
** Furthermore the commentators speak of two Kitab (Book/record) in the Knowledge of Allah:
1. The Book of the Knowledge of Allah
2. The Book that is kept of the deeds of the slaves.
Imam Bukhari reported in the “Book of Qur’an Commentary” ( Kitab al-Tafsir) and so did others that Umm al Kitab is the designation of Surat al Fatiha, as it is the Opening of the Qur’an. Imam Suyuti adds to this that al Fatiha contains all the subsequent knowledge that will be explicated in the rest of the Qur’anic Chapters ( Surah).
And Allah knows best.