Sa'id b. Jubair reported:
I said to Ibn Abbas: Will the repentance of that person be accepted who kills a believer intentionally? He said: No. I recited to him this verse of Sura al-Furqan (xix.):" And those who call not upon another god with Allah and slay not the soul which Allah has forbidden except in the cause of justice" to the end of the verse. He said: This is a Meccan verse which has been abrogated by a verse revealed at Medina:" He who slays a believer intentionally, for him is the requital of Hell-Fire where he would abide for ever," and in the narration of Ibn Hisham (the words are): I recited to him this verse of Sura al-Furqan:" Except one who made repentance."
Reference: Sahih Muslim 3023 e
In-book reference: Book 56, Hadith 23
USC-MSA web (English) reference(deprecated numbering scheme): Book 43, Hadith 7173