Source : Tafseer Ibn Kathir of Surah Al-Kahf
And never say of anything, “I shall do such and such thing tomorrow.” 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.” – Surah Kahf: (23, 24 ayah).
When Allah’s Messenger (sallalahu alaihe wa-sallam) declared Allah’s Oneness and proclaimed to be the Messenger of Allah (sallalahu alaihe wa-sallam) and revelation of Divine Message, the disbeliveers of Makkah sent some men to Jewish rabbis in al-Medina and said: “Ask them (the rabbis) about Muhammad (sallalahu alaihe wa-sallam), and describe him to them, and tell them what he is saying.They are the people of the first Book, and they have more knowledge of the Prophets than we do.’ So they set out for al-Medina, they asked the Jewish rabbis about the Messenger of Allah (sallalahu alaihe wa-sallam). They described him to them and told them some of his teachings.
The rabbis said: ‘Ask him about three things which we will tell you to ask, if he answers them then he is a Prophet who has been sent (by Allah); if he does not, then he is saying things that are not true, in which case how you will deal with him will be up to you. Ask him about some young men in ancient times, what was their story? For there is a strange and wondrous tale. Ask him about a man who traveled a great deal and reached the east and the west of the earth. What was his story? And ask him about he Ruh (soul or spirit) what is it?
If he tells you about these things, then he is a Prophet, so follow him, but if he does not tell you, then he is a man who is making things up, so deal with him as you see fit.’ So, the men came back to Makkah and said: ‘O people! We have come to you with a decisive solution which will put an end to the problem between you and Muhammad (sallalahu alaihe wa-sallam). The Jewish rabbis told us to ask him about some matters,’ and they told them what they were. Then they came to the Messenger of Allah (sallalahu alaihe wa-sallam) and said: ‘O Muhammad (sallalahu alaihe wa-sallam) tell us,’ and they asked him about the things they had been told to ask.
The Messenger of Allah (sallalahu alaihe wa-sallam) said: “I will tell you tomorrow about what you have asked me.” But he (sallalahu alaihe wa-sallam) did not say, ‘If Allah Wills.’
Allah Messenger (sallalahu alaihe wa-sallam) never spoke anything except with revelation. So, he waited for the revelation from Allah but fifteen days passed away without any revelation concerning those matter which the disbelievers asked, neither did Jibreel (alaihis-salaam) come to him. The people of Makkah started to doubt him and said: ‘Muhammad (sallalahu alaihe wa-sallam) 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 (sallalahu alaihe wa-sallam) felt sad because of the delay in revelation and was grieved by what the people of Makkah were saying about him.
Then Jibreel (alaihis-salaam) came with the revelation of Soorah al-Kahf, which contained answers to the questions and also the above mentioned verse explaining the correct etiquette when determining to do something in the future.
Determining to do something in the future should always be attributed to the Will of Allah, Who is the Knower of the Unseen and Who Alone Knows what was and what is yet to happen and what is not to be.
It is reported in Saheeh al-Bukharee from Abu Hurayrah (radhiyallahu anhu), who said: “The Messenger of Allah (sallalahu alaihe wa-sallam) said: “Sulaiman Ibn Dawood (alaihimus-salaam) said: ‘Tonight I will go around to all my seventy wives (according to some reports it is ninety or one hundred wives) 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 narration, an 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 (sallalahu alaihe wa-sallam) said: “By the One, in Whose Hands 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.” [Saheeh Muslim (vol. 3, no. 1275)]
So, if one determines to do something in the future or even takes an oath, he should say, ‘If Allah Wills’ or ‘Insha’Allah.’ If one forgets to say “Insha’Allah” then he should say when he remembers it even if it is a year later, as Ibn Abbas (radhiallahu anhu) explained.