Al-Isra' - 17:33

وَلاَ تَقْتُلُواْ النَّفْسَ الَّتِي حَرَّمَ اللّهُ إِلاَّ بِالحَقِّ وَمَن قُتِلَ مَظْلُومًا فَقَدْ جَعَلْنَا لِوَلِيِّهِ سُلْطَانًا فَلاَ يُسْرِف فِّي الْقَتْلِ إِنَّهُ كَانَ مَنْصُورًا
[17:33]
And do not kill any one whom Allah has forbidden, except for a just cause, and whoever is slain unjustly, We have indeed given to his heir authority, so let him not exceed the just limits in slaying; surely he is aided.
Nor take life - which Allah has made sacred - except for just cause. And if anyone is slain wrongfully, we have given his heir authority (to demand qisas or to forgive): but let him not exceed bounds in the matter of taking life; for he is helped (by the Law).
And slay not the life which Allah hath forbidden save with right. Whoso is slain wrongfully, We have given power unto his heir, but let him not commit excess in slaying. Lo! he will be helped.