﴿ إِلَّآ أَن يَشَآءَ ٱللَّهُ ۚ وَٱذْكُر رَّبَّكَ إِذَا نَسِيتَ وَقُلْ عَسَىٰٓ أَن يَهْدِيَنِ رَبِّى لِأَقْرَبَ مِنْ هَٰذَا رَشَدًۭا ﴾

“without (adding], "if God so wills." And if thou shouldst forget [thyself at the time, and become aware of it later], call thy Sustainer to mind and say: "I pray that my Sustainer guide me, even closer than this, to a consciousness of what is right!"”

(Except if Allah will) unless you second it by saying: if Allah will. (And remember thy Lord) without exception (when thou forgettest) even if it is after a while, (and say: It may be that my Lord guideth me) and lead me (unto a nearer way of truth than this) a more correct and certain way than this. This verse was revealed about the Prophet (pbuh) for he said to the idolaters of Mecca when they asked him about the spirit: " tomorrow, I will answer you " , and he forgot to say " if Allah wills ".