It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him); he said: A man came to the Prophet (peace and blessings be upon him) and said: O Messenger of Allah, this man killed my brother. He (peace and blessings be upon him) said: "How did you kill him?" He said: I struck his head with an axe; I did not intend to kill him. He (peace and blessings be upon him) said (to the killer): "Do you have enough wealth to pay his blood money (diyah)?" He said: No. So the Prophet (peace and blessings be upon him) said (to the slain man's brother): "Take him." He went out with him to kill him, and the Prophet (peace and blessings be upon him) said: "If he kills him, he will be like him." When he heard this, he brought him back and said: Here he is; now command concerning him whatever you wish. The Prophet (peace and blessings be upon him) said: "Let him go."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18242