﴿ إِلَّا مَا شَآءَ ٱللَّهُ ۚ إِنَّهُۥ يَعْلَمُ ٱلْجَهْرَ وَمَا يَخْفَىٰ ﴾

“save what God may will [thee to forget] - for, verily, He [alone] knows all that is open to [man's] perception as well as all that is hidden [from it]”

except what God may will that you forget by abrogating its recitation and its ruling — the Prophet s used to recite aloud while Gabriel recited fearing to forget and so it is as though it was said to him ‘Do not hasten in it; you will not forget and so do not weary yourself by reciting it aloud’; assuredly He exalted be He knows what is overt of words and deeds and what is hidden of both of these.