Surah Al-Fat'h (The Victory), Verse 16

قُلْ لِّلْمُخَلَّفِیْنَ مِنَ الْاَعْرَابِ سَتُدْعَوْنَ اِلٰی قَوْمٍ اُولِیْ بَاْسٍ شَدِیْدٍ تُقَاتِلُوْنَهُمْ اَوْ یُسْلِمُوْنَ فَاِنْ تُطِیْعُوْا یُؤْتِكُمُ اللّٰهُ اَجْرًا حَسَنًا وَاِنْ تَتَوَلَّوْا كَمَا تَوَلَّیْتُمْ مِّنْ قَبْلُ یُعَذِّبْكُمْ عَذَابًا اَلِیْمًا ۟

Qul lilmukhallafeena minal A'raabi satud'awna ilaa qawmin ulee baasin shadeedin tuqaati loonahum aw yuslimoona fa in tutee'oo yu'tikumul laahu ajran hasananw wa in tatawallaw kamaa tawallaitum min qablu yu'azzibkum 'azaaban aleemaa

“Say to the desert Arabs who lagged behind: "Ye shall be summoned (to fight) against a people given to vehement war: then shall ye fight, or they shall submit. Then if ye show obedience, Allah will grant you a goodly reward, but if ye turn back as ye did before, He will punish you with a grievous Penalty."”

Quran 48:16 (Yusuf Ali Translation)