It is not (permissible) for a believer to kill a believer except by mistake; and whoever kills a believer by mistake, he should set free a believing slave and compensation payment should be given to the family of the slain, unless they remit the compensation payment as alms. But if he (who killed a man by mistake) is from a tribe hostile to you and he is a believer, shall free a believing slave; and if he is from a tribe between whom and you there is a covenant, the compensation should be paid to his people along with the freeing of a believing slave; but he who can not fulfill these should fast for two months successively in order to seek repentance from Allah. And Allah is All-Knowing, All-Wise.
And whoever kills a believer intentionally, then his punishment is Hell; he shall abide in it forever; Allah has sent His Wrath on him and cursed him and He has prepared for him a great torment.
O you who believe (who are âmenû)! When you go out (to fight) in the Way of Allah, investigate to bring out into the open (in order to distinguish the believers from non-believers) and do not say any one who greets you (surrenders) aspiring for the goods of worldly life: you are not a believer, seeking the perishable goods of the worldly life. But with Allah there are abundant booties. You too were such before, then Allah conferred a blessing upon you; therefore investigate and bring out; surely Allah is Well-Aware of what you do.