“I will tell you tomorrow”, said the Messenger of Allah [SAWS] when he was asked about the story of the people of the Cave (AlKahf).
He [SAWS] forgot to say in sha Allah. The result? Allah [SWT] delayed the revelation by 15 days. Then, Allah [SWT] revealed the ayaat and instructed His Prophet [SAWS] on the proper etiquettes when scheduling a task for the future:
وَلَا تَقُولَنَّ لِشَيْءٍ إِنِّي فَاعِلٌ ذَلِكَ غَدًا
And never say of anything, “I shall do such and such thing tomorrow.”
إِلَّا أَن يَشَاء اللَّهُ وَاذْكُر رَّبَّكَ إِذَا نَسِيتَ وَقُلْ عَسَى أَن يَهْدِيَنِ رَبِّي لِأَقْرَبَ مِنْ هَذَا رَشَدًا
Except (with the saying), “If Allah wills!” And remember your Lord when you forget and say: “It may be that my Lord guides me to a nearer way of truth than this.”
So here’s a rule of thumb:
Never, EVER say “I’m gonna do this or that tomorrow”, unless if prefixed or suffixed with “in sha Allah (if Allah wills)”. Because ONLY Allah [SWT] knows when the time comes whether or not you’ll be able to do what you intend to do.
This is the lesson that Allah [SWT] taught to His beloved Prophet [SAWS]. The question is, how will we learn this lesson: by reading the Quran and taking heed, or by having pain inflicted upon us to remind us of this lesson?
WAllahu alim (And Allah knows best).