Surah An-Nisaa 4:92

And never is it for a believer to kill a believer except by mistake. And whoever kills a believer by mistake - then the freeing of a believing slave and a compensation payment presented to the deceased's family [is required] unless they give [up their right as] charity. But if the deceased was from a people at war with you and he was a believer - then [only] the freeing of a believing slave; and if he was from a people with whom you have a treaty - then a compensation payment presented to his family and the freeing of a believing slave. And whoever does not find [one or cannot afford to buy one] - then [instead], a fast for two months consecutively, [seeking] acceptance of repentance from Allah. And Allah is ever Knowing and Wise.