(Prophet!) say to those of the desert Arabs who (contrived to be) left behind, `You shall soon be called upon (to take up arms) against a people (- the Byzantine and the Persians) who are fighters of mighty valour. Then you shall fight until they submit. Allâh will grant you a goodly reward if you obey. But if you turn your backs, as you did before, He will punish you, a woeful punishment.'