Sahih Muslim Book 16, Hadith Number 4165.

Chapter : The confession of murder is held valid and the person whose man is killed is entitled to get retribution, the offender has a right to beg for remission.

‘Alaqama b. Wa’il reported on the authority of his father that a person was brought to the Messenger of Allah (may peace be upon him) who had killed another person, and the heir of the person slain had dragged him (to the Holy Prophet) with a strap around his neck. As he turned away Allah’s Messenger (may peace be upon him) said: The killer and the killed are (doomed) to fire. A person came to the other person (the heir of the deceased) and he reported to him the words of the Messenger of Allah (may peace be upon him), and so he let him off. Isma’il b. Salim said: I made a mention of it to Habib b. Abu Thabit and he said: Ibn Ashwa’ reported to me that Allah’s Apostle (may peace be upon him) had asked him to pardon him, but he refused.