Volume 4, Book 52, Number 74 :
Narrated by Abu Huraira (radiallaahu `anhu)
Allah’s Apostle (sallallaahu `alayhi wasallam) said, “Once Solomon, son of David said, ‘(By Allah) Tonight I will have sexual intercourse with one hundred (or ninety-nine) women each of whom will give birth to a knight who will fight in Allah’s Cause.’ On that a companion of his said inshaAllah (i.e. if Allah wills) but he (Solomon) did not say, ‘Allah willing.’ Therefore only one of those women conceived and gave birth to a half-man. By Him in Whose Hands Muhammad’s life is, if he had said, “Allah willing’, (he would have begotten sons) all of whom would have been knights striving in Allah’s Cause.”
وَلَا تَقُولَنَّ لِشَيْءٍ إِنِّي فَاعِلٌ ذَٰلِكَ غَدًا
إِلَّا أَن يَشَاءَ اللَّهُ ۚ وَاذْكُر رَّبَّكَ إِذَا نَسِيتَ وَقُلْ عَسَىٰ أَن يَهْدِيَنِ رَبِّي لِأَقْرَبَ مِنْ هَٰذَا رَشَدًا
And never say of anything, “Indeed, I will do that tomorrow”, except [when adding], “If Allah wills.” And remember your Lord when you forget [it] and say, “Perhaps my Lord will guide me to what is nearer than this to right conduct.” [18:23-24]
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 disbelievers 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-Madina, 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.
Taken from AlQuranClasses.com
An interesting story on the subject: Radwan Goes to Buy a Horse