﴿ يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ لَا تَكُونُوا۟ كَٱلَّذِينَ كَفَرُوا۟ وَقَالُوا۟ لِإِخْوَٰنِهِمْ إِذَا ضَرَبُوا۟ فِى ٱلْأَرْضِ أَوْ كَانُوا۟ غُزًّۭى لَّوْ كَانُوا۟ عِندَنَا مَا مَاتُوا۟ وَمَا قُتِلُوا۟ لِيَجْعَلَ ٱللَّهُ ذَٰلِكَ حَسْرَةًۭ فِى قُلُوبِهِمْ ۗ وَٱللَّهُ يُحْىِۦ وَيُمِيتُ ۗ وَٱللَّهُ بِمَا تَعْمَلُونَ بَصِيرٌۭ ﴾

“O you who have attained to faith! Be not like those who are bent on denying the truth and say of their brethren [who die] after having set out on a journey to faraway places or gone forth to war, "Had they but remained with us, they would not have died," or, "they would not have been slain" - for God will cause such thoughts to become a source of bitter regret in their hearts, since it is God who grants life and deals death. And God sees all that you do.”

Please check ayah 3:158 for complete tafsir.