Al-Baqarah 2:178 يا ايها الذين امنوا كتب عليكم القصاص في القتلى الحر بالحر والعبد بالعبد والانثى بالانثى فمن عفي له من اخيه شيء فاتباع بالمعروف واداء اليه باحسان ذالك تخفيف من ربكم ورحمة فمن اعتدى بعد ذالك فله عذاب اليم ١٧٨

O you who believe! Al-Qisâs (the Law of Equality in punishment) is prescribed for you in case of murder: the free for the free, the slave for the slave, and the female for the female. But if the killer is forgiven by the brother (or the relatives, etc.) of the killed against blood-money, then adhering to it with fairness and payment of the blood-money to the heir should be made in fairness. This is an alleviation and a mercy from your Lord. So after this whoever transgresses the limits (i.e. kills the killer after taking the blood-money), he shall have a painful torment.