Sa'id bin Jubayr (may Allah have mercy on him) said: I asked Ibn Abbas (may Allah be pleased with him) about this verse: 'And whoever kills a believer intentionally, his recompense is Hell' [Surah al-Nisa: 93], and he said: There is no repentance for him. And I asked about this verse: 'And those who do not call upon another god along with Allah' [Surah al-Furqan: 68] up to His words 'except whoever repents and believes' [Surah al-Furqan: 70], and he said: This was in the time of ignorance (Jahiliyyah).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / 15826