Tafsir of Al-Kahf 18:23

"And do not say of anything..." (meaning: for any matter you intend to undertake), "...'Indeed, I will do that [thing] tomorrow'..." (meaning: at any time in the future, absolutely. This is an emphasis on what the active participle [‘fa’il’] indicates, based on the principle that it is essentially [denotative] of the future).

Included in this [meaning] is ‘tomorrow’ in the sense of the day immediately following your current day, which is the primary and immediate implication. For the verse was revealed when the Quraysh asked the Prophet (peace and blessings be upon him) about the Spirit, the People of the Cave, and Dhu al-Qarnayn, and he (peace and blessings be upon him) replied, "I will tell you tomorrow," without making an exception [by saying 'Insha'Allah']. Revelation was then withheld from him for fifteen days, according to what is narrated from Ibn Ishaq; it is also said [it was withheld for] three days, or forty days. This caused him (peace and blessings be upon him) great distress, and the Quraysh accused him of falsehood—may he be far exalted above that.

Many have permitted that it remain upon its primary, immediate meaning, and that [the prohibition of] what follows it [in terms of time] is known by way of textual inference (dalalat al-nass).

It has been argued against this that what follows it is not the same in terms of the underlying reason for the prohibition, which is the possibility of an obstacle, for as time extends, obstacles may be removed or lessened. But this is of no account, because the "obstacle" [mentioned as a condition for saying 'Insha'Allah'] includes [the possibility of] death, and the likelihood [of obstacles] in an extended period of time is even stronger.