The Women - 4:92
A believer shall not kill a believer except by accident. Whoever kills a believer by accident must release a believing slave and pay a diet to the family of the deceased. Unless they give up the diet and accept it as charity. If it is a believer from a community that is at war with you, then you must release a believing slave. But if the deceased belongs to a community with whom you have an agreement, then you must give his family a diet and release a believing slave. Whoever cannot find the money or a slave to release should fast for two months in a row so that Allah may accept his repentance. Allah is All-Knowing, All-Wise.
A believer who is at war with you:
The Apartments/49:9
And if two groups of the believers fight against each other, reconcile them. If one of them attacks the other, fight the aggressor until he returns to the command of Allah. If they return, then settle their differences impartially and do justice. Allah loves those who do justice.