(It is not for a believer) it is not allowed for a believer, meaning here 'Ayyash Ibn Abi Rabi'ah (to kill a believer) Harith Ibn Zayd ((unless (it be) by mistake) even if it be by mistake. (He who hath killed a believer by mistake must set free a believing slave) a slave who believes in Allah and His Messenger, (and pay the blood money) in full (to the family of the slain) to the next of kin of the victim who have the right to demand retaliation, (unless they remit it as a charity) unless the next of kin of the victim remit the blood money to the killer as a charity. (If he) the victim (be of a people hostile unto you) who are at war with you, (and he) i.e. the victim (is a believer, then (the penance is) to set free a believing slave) the killer should set free a slave who believes in Allah and His Messenger and so does not have to pay the blood money. Harith belonged to a people who were at war with the Messenger of Allah (pbuh). (And if he) the victim (cometh of a folk between whom and you there is a covenant) a treaty and peace, (then the blood money must be paid) in full (unto his folk) to the next of kin of the victim (and (also) a believing slave must be set free) a slave who believes in the divine Oneness of Allah. (And whoso hath not the wherewithal) to set free a believing slave (must fast two consecutive months) continuously. (A penance from Allah) if he does this, Allah will forgive him for killing a believer by mistake. (Allah is Knower) of him who kills by mistake, (Wise) in that which he ought to do.
