Muslims Should Not Kill Muslims

It is not for a believer to kill a believer unless (it be) by mistake. He who hath killed a believer by mistake must set free a believing slave, and pay the blood- money to the family of the slain, unless they remit it as a charity. If he (the victim) be of a people hostile unto you, and he is a believer, then (the penance is) to set free a believing slave. And if he cometh of a folk between whom and you there is a covenant, then the blood-money must be paid unto his folk and (also) a believing slave must be set free. And whoso hath not the wherewithal must fast two consecutive months. A penance from Allah. Allah is Knower, Wise. Whoso slayeth a believer of set purpose, his reward is hell for ever. Allah is wroth against him and He hath cursed him and prepared for him an awful doom. (4:92-93)