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Surah 3. Aal-Imran
Ayah 156 - 158
156يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ لَا تَكُونُوا۟ كَٱلَّذِينَ كَفَرُوا۟ وَقَالُوا۟ لِإِخْوَٰنِهِمْ إِذَا ضَرَبُوا۟ فِى ٱلْأَرْضِ أَوْ كَانُوا۟ غُزًّى لَّوْ كَانُوا۟ عِندَنَا مَا مَاتُوا۟ وَمَا قُتِلُوا۟ لِيَجْعَلَ ٱللَّهُ ذَٰلِكَ حَسْرَةً فِى قُلُوبِهِمْ ۗ وَٱللَّهُ يُحْىِۦ وَيُمِيتُ ۗ وَٱللَّهُ بِمَا تَعْمَلُونَ بَصِيرٌ
157وَلَئِن قُتِلْتُمْ فِى سَبِيلِ ٱللَّهِ أَوْ مُتُّمْ لَمَغْفِرَةٌ مِّنَ ٱللَّهِ وَرَحْمَةٌ خَيْرٌ مِّمَّا يَجْمَعُونَ
158وَلَئِن مُّتُّمْ أَوْ قُتِلْتُمْ لَإِلَى ٱللَّهِ تُحْشَرُونَ
[156-158] O Believers, do not behave like disbelievers: when their brethren are on a journey or take part in a battle, (and ever meet with some accident) they say, "Had they stayed with us, they would not have died nor would have been slain." Allah makes such things113 a cause of grief and anguish in their hearts: for in fact, it is Allah who ordains both life and death; Allah is watching all that you do. If you are slain or die in the way of Allah, you will get Allah's forgiveness and mercy which are far better than all the things they amass. Anyhow, you shall be gathered together before Allah whether you die or are slain.
113These were their vain regrets which were the result of their wrong thinking. The fact is that the decrees of God are inevitable. Such regrets produce only grief and anguish in the hearts of those who do not trust in Allah but in their own plans and designs. However, when their efforts also end in utter failure, they wring their hands, saying, "Oh! had it been thus and thus, the result would have been so and so."