Sayyiduna Uqbah bin Malik (may Allah be pleased with him), who was from the clan of Nasr bin Asim al-Laythi, narrates that the Messenger of Allah (peace and blessings be upon him) sent out a small detachment, and they attacked a people. A man broke away from the people, and a man of the detachment pursued him with a drawn sword. The man who had broken away said: I am a Muslim. But he paid no heed to it and struck him and killed him. When the matter reached the Prophet (peace and blessings be upon him), he (peace and blessings be upon him) spoke severely about him. The killer said: O Messenger of Allah, he only said that to save himself from being killed. He (peace and blessings be upon him) turned away from him three times, and he repeated the same words. Then he (peace and blessings be upon him) turned towards him, and displeasure could be recognised in his (peace and blessings be upon him) face, and he (peace and blessings be upon him) said: "Allah, Lord of Might, refuses (to accept any excuse from) the one who kills a believer." He (peace and blessings be upon him) said it three times.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18272