سورة Muhammad - الآية 4

When you encounter the disbelievers [in battle], strike [their] necks until you have thoroughly subdued them, then bind the captives firmly[2]. Later [you may free them] as a favor[3] or for ransom until the war comes to an end[4]. That [is Our command], if Allah had willed, He could have subjected them to punishment, but [He ordered you to fight] to test some of you by means of others. Those who are killed in the way of Allah, He will never render their deeds void.