Saying "If Allah wills" when determining to do Something in the Future

Here Allah, may He be glorified, shows His Messenger the correct etiquette when determining to do something in the future; this should always be referred to the will of Allah, the Knower of the Unseen, Who knows what was and what is yet to be and what is not to be, and how it will be if it is to be. It was recorded in the Two Sahihs that Abu Hurayrah said that the Messenger of Allah ﷺ said:

«قَالَ سُلَيْمَانُ بْنُ دَاوُدَ عَلَيْهِمَا السَّلَامُ: لَأَطُوفَنَّ اللَّيْلَةَ عَلَى سَبْعِينَ امْرَأَةً وَفِي رِوَايَةٍ: تِسْعِينَ امْرَأَةً، وَفِي رِوَايَةٍ: مِائَةِ امْرَأَةٍ تَلِدُ كُلُّ امْرَأَةٍ مِنْهُنَّ غُلَامًا يُقَاتِلُ فِي سَبِيلِ اللهِ، فَقِيلَ لَهُ وَفِي رِوَايَةٍ قَالَ لَهُ الْمَلَكُ: قُلْ إِنْ شَاءَ اللهُ، فَلَمْ يَقُلْ، فَطَافَ بِهِنَّ فَلَمْ تَلِدْ مِنْهُنَّ إِلَّا امْرَأَةٌ وَاحِدَةٌ نِصْفَ إِنْسَانٍ، فَقَالَ رَسُولُ اللهِصلى الله عليه وسلّم: وَالَّذِي نَفْسِي بِيَدِهِ، لَوْ قَالَ إِنْ شَاءَ اللهُ لَمْ يَحْنَثْ، وَكَانَ دَرَكًا لِحَاجَتِه»

Saying 'Insha'Allah' on doing something in the future

The background of the revelation of the first two verses has been reported in Lubab from Sayyidna ` Abdullah ibn ` Abbas ؓ in the following manner. When the people of Makkah acting as coached by the Jews, asked the Holy Prophet ﷺ about the story of Ashab al-Kahf etc., he promised to answer that next day without having said Insha'Allah. The least shortcoming issuing forth from the close ones brings some or the other notice of caution. Therefore, no revelation came for the next fifteen days. Naturally, the Holy Prophet ﷺ was grieved and the Mushriks of Makkah had their opportunity to laugh and ridicule. After this interval of fifteen days, when the answer to the questions asked was revealed, these two verses were revealed along with it as a measure of guidance. They told him that, should he need to say something about what he was going to do the next day, he should invariably affirm it by saying Insha'Allah, for everything depends on the intention and will of Allah Ta` ala. These two verses, it will be noted, have been introduced at the end of the story of the People of Kahf.