سورة An-Nisa ( The Women ) - الآية 92

And it does not behoove a believer to kill a believer except by mistake, and whoever kills a believer by mistake, he should free a believing slave, and blood-money should be paid to his people unless they remit it as alms; but if he be from a tribe hostile to you and he is a believer, the freeing of a believing slave (suffices), and if he is from a tribe between whom and you there is a convenant, the blood-money should be paid to his people along with the freeing of a believing slave; but he who cannot find (a slave) should fast for two months successively: a penance from Allah, and Allah is Knowing, Wise.