Alqamah bin Wa'il al-Hadrami (may Allah have mercy on him) narrates from his father Wa'il bin Hujr (may Allah be pleased with him), from the Prophet (peace and blessings be upon him), that a man was brought to him (peace and blessings be upon him) who had killed someone, and the killer was handed over to the guardian of the slain man so that he might kill him. When he set off with him, with a rope around his neck, the Prophet (peace and blessings be upon him) said to those sitting with him: "The killer and the slain are both in the Fire." A man went to that guardian and told him these words of the Messenger of Allah (peace and blessings be upon him), so he let him go. I saw him when he let him go; he was walking away dragging his rope. Isma'il said: I mentioned this to Habib bin Abi Thabit, and he related from Ibn Ashwa' that the Messenger of Allah (peace and blessings be upon him) had ordered that man to forgive, but it seems he did not do so; then when he was told what the Messenger of Allah (peace and blessings be upon him) had said, he let him go.
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18227