Abd al-Rahman bin al-Qasim (may Allah have mercy on him) narrates from his father that al-Harith bin Zayd (may Allah be pleased with him) was a bitter enemy of the Prophet (peace and blessings be upon him). He became a Muslim, but Ayyash (may Allah be pleased with him) did not know of his Islam, so Ayyash (may Allah be pleased with him) came and killed him. Then Allah revealed this verse: "And it is not for a believer to kill a believer" [Surah al-Nisa: 92]
Imam al-Shafi'i (may Allah have mercy on him) said: Allah has stated in His Book the prescribed diyah due from the killer of a believer, and He made it clear through the tongue of His Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16142