So when you meet in battle those who disbelieve, smite the necks; then, when you have overcome them, make (them) prisoners, and afterwards (set them free) as a favour or for ransom till the war lay down its burdens. That (shall be so). And if Allah please, He would certainly exact retribution from them, but that He may try some of you by means of others. And those who are slain in the way of Allah, He will never allow their deeds to perish.