قُل لِّلْمُخَلَّفِينَ مِنَ ٱلْأَعْرَابِ سَتُدْعَوْنَ إِلَىٰ قَوْمٍ أُو۟لِى بَأْسٍ شَدِيدٍ تُقَٰتِلُونَهُمْ أَوْ يُسْلِمُونَ ۖ فَإِن تُطِيعُوا۟ يُؤْتِكُمُ ٱللَّهُ أَجْرًا حَسَنًا ۖ وَإِن تَتَوَلَّوْا۟ كَمَا تَوَلَّيْتُم مِّن قَبْلُ يُعَذِّبْكُمْ عَذَابًا أَلِيمًا

Say to those of the dwellers of the desert who were left behind: You shall soon be invited (to fight) against a people possessing mighty prowess; you will fight against them until they submit; then if you obey, Allah will grant you a good reward; and if you turn back as you turned back before, He will punish you with a painful punishment.