Alqamah bin Wa'il narrates from his father that a man was brought to the Prophet (peace and blessings be upon him) so that retaliation might be taken from him; there was a strap around his neck by which he was being dragged. When they had gone, he (peace and blessings be upon him) said: "The killer and the killed are both in Hell." A man then met that man and told him what the Prophet (peace and blessings be upon him) had said, so he let him go. Isma'il said: I mentioned this to Habib bin Abi Thabit. He replied: Ibn Ashwa' told me that the Prophet (peace and blessings be upon him) asked him: "Do you pardon?" and he had refused.
And Abu Awanah narrated it from Isma'il, who said: I mentioned it to Ibn Ashwa', and Ibn Ashwa' said: I mentioned this to Habib, and he said that the Prophet (peace and blessings be upon him) had ordered him to pardon.
Sa'id bin Jubayr (may Allah have mercy on him) narrated from the Prophet (peace and blessings be upon him) in mursal form that the man said: O Messenger of Allah (peace and blessings be upon him)! He killed my brother, so he is in Hell; if I kill him, will I be like him? He (peace and blessings be upon him) said: "He killed your brother, so he is of the people of Hell; and if you disobey my command, you too will go to Hell."
And one view is that he said this because the killer had said: By Allah, I did not intend to kill him. And in the hadith of Sayyiduna Abu Hurayrah (may Allah be pleased with him) it is: If he is truthful and you kill him while knowing that he is truthful, you will become like him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16052