Illa ma shaa Allahu innahu yaAAlamu aljahra wama yakhfa (surah Al Ala: 7)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(87:7) except what Allah should wish. *8 He knows all that is manifest and all that is hidden. *9

Illaa maa shaaa`al laah; innahoo ya`lamul jahra wa maa yakhfaa

*8) This sentence can have two meanings: first, "that the preservation of the entire Qur'an word for word in your memory is not a manifestation of your own power but the result of Allah Almighty's bounty and grace; otherwise if Allah so willed, He could cause it to be forgotten. " This same theme has been expressed at another place in the Qur'an, thus: "O Muhammad! We may, if We so will, take back from you all of what We have revealed to you." (Bani Isra'il: 86). Another meaning also can be: "Your forgeting something temporarily, or your forgetting a verse or a word occasionally is an axception froth this promise. What has been promised is: `You will not forget any word of the Qur'an permanently'." This meaning is confirmed by the following tradition of Bukhari: Once while leading the Fajr Prayer the Holy Prophet (upon whom be peace) happened to omit a verse during the recitation. After the Prayer hadrat Ubayy bin Ka`b asked if the verse had been abrogated. The Holy Prophet replied that he had forgotten to recite it.
*9) Although the words are general, and they mean that Allah knows everything, whether it is open or hidden, yet in the context in which they occur, they seem to mean: "O Prophet, your reciting the Qur'an along with Gabriel (peace be on him) is known to Allah and your fear for which you do so is also in Allah's knowledge. " Therefore, the Holy Prophet is being assured that he will not forget it.