﴿ إِلَّا تَنفِرُوا۟ يُعَذِّبْكُمْ عَذَابًا أَلِيمًۭا وَيَسْتَبْدِلْ قَوْمًا غَيْرَكُمْ وَلَا تَضُرُّوهُ شَيْـًۭٔا ۗ وَٱللَّهُ عَلَىٰ كُلِّ شَىْءٍۢ قَدِيرٌ ﴾

“If you do not go forth to war [in God's cause], He will chastise you with grievous chastisement, and will place another people in your stead - whereas you shall in no wise harm Him: for, God has the power to will anything.”

(If ye go not forth) with your Prophet at the Battle of Tabuk (He will afflict you with a painful doom) in this world and in the Hereafter, (and will choose instead of you a folk other than you) who are better and more obedient than you. (Ye cannot harm Him all) i.e. by staying back you cannot harm Allah. (Allah is Able to do all things) whether it is punishment or reward.