Sayyiduna Anas (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Allah Almighty has refused to accept the repentance of the killer of a believer."
Hadith Referenceسلسله احاديث صحيحه / الفتن و اشراط الساعة والبعث / 3742