﴿ وَإِذَا ضَرَبْتُمْ فِى ٱلْأَرْضِ فَلَيْسَ عَلَيْكُمْ جُنَاحٌ أَن تَقْصُرُوا۟ مِنَ ٱلصَّلَوٰةِ إِنْ خِفْتُمْ أَن يَفْتِنَكُمُ ٱلَّذِينَ كَفَرُوٓا۟ ۚ إِنَّ ٱلْكَٰفِرِينَ كَانُوا۟ لَكُمْ عَدُوًّۭا مُّبِينًۭا ﴾

“AND WHEN you go forth [to war] on earth, you will incur no sin by shortening your prayers if you have reason to fear that those who are bent on denying the truth might suddenly fall upon you: for, verily, those who deny the truth are your open foes.”

(And when ye go forth in the land) when you travel, (it is no sin for you to curtail (your) worship) curtail the prayer that a sedentary Muslim would usually perform (if ye fear) if you know (that those who disbelieve may attack you) may kill you while in prayer. (In truth the disbelievers are an open enemy to you) this is what is called the prayer of fear (salat al-khawf).