In keeping with his human nature, when Rasulullah (sallallahu ‘alaihi wasallam) forgot to say insha Allah, the following verses of the Qur’aan Majeed were revealed:
وَ لَا تَقُوۡلَنَّ لِشَایۡءٍ اِنِّیۡ فَاعِلٌ ذلِکَ غَدًا ﴿ۙ۲۳﴾ اِلَّاۤ انۡ یَّشَآءَ اللّٰه
And never say about anything, “I will do it tomorrow,” except by adding insha Allah to it, and when you forget it, then remember your Lord (by saying insha Allah, as this will make amends for this forgetfulness). (Surah Kahf v. 23-24)
It is for this reason that Hazrat ‘Abdullah bin ‘Abbaas (radhiyallahu ‘anhuma) would say that even if a person remembers to say insha Allah after a whole year passes, he should still say it at that time, as this will compensate for his mistake and forgetfulness.
When one omits mentioning the will of Allah Ta‘ala, and relies on his own will by mentioning, “I will do this tomorrow,” then Allah Ta‘ala dislikes it. This is why, if a person forgets to say insha Allah presently, he may make amends for this act of forgetfulness by saying insha Allah whenever he remembers, as this will compensate for his forgetfulness.
(Extracted from Seeratul Mustafa 1/195)