If Allah Wills

{8:23-24}And do not say of anything: Surely I will do it tomorrow.
Unless Allah wills; and remember your Lord when you forget and say: Maybe my Lord will guide me to a nearer course to the right than this.

Ibn Kathir explains that when The Messenger of Allah was asked by Quraish about the people of the cave he said I will tell you tomorrow about what you have asked me but he did not say `If Allah wills.' So they went away, and the Messenger of Allah stayed for fifteen days without any revelation from Allah concerning that, and Jibril, peace be upon him, did not come to him either. The people of Makkah started to doubt him, and said, `Muhammad promised to tell us the next day, and now fifteen days have gone by and he has not told us anything in response to the questions we asked.

The Messenger of Allah felt sad because of the delay in revelation, and was grieved by what the people of Makkah were saying about him. Then Jibril came to him from Allah with the Surah about the companions of Al-Kahf, which also contained a rebuke for feeling sad about the idolators.

It was recorded in the Two Sahihs that Abu Hurayrah said that the Messenger of Allah said: Sulayman bin Dawud (peace be upon them both) said: "Tonight I will go around to seventy women [according to some reports, it was ninety or one hundred women] so that each one of them will give birth to a son who will fight for the sake of Allah.''

It was said to him, [according to one report, the angel said to him] "Say: `If Allah wills'", but he did not say it. He went around to the women but none of them gave birth except for one who gave birth to a half-formed child.) The Messenger of Allah said, (By the One in Whose hand is my soul, had he said, "If Allah wills,'' he would not have broken his oath, and that would have helped him to attain what he wanted. ) According to another report, (They would all have fought as horsemen in the cause of Allah.)

…and remember your Lord when you forget…

It was said that this means, if you forget to say "If Allah wills", then say it when you remember. This was the view of Abu Al-`Aliyah and Al-Hasan Al-Basri. Hushaym reported from Al-A`mash from Mujahid that concerning a man who swears an oath, Ibn `Abbas said "He may say `If Allah wills' even if it is a year later. This was also the view of Ibn Jarir, but he stated that this does not make up for breaking the oath or mean that one is no longer obliged to offer expiation. The Sunnah is that he should say it, so that he will still be following the Sunnah of saying "If Allah wills'', even if that is after breaking his oath.