Sa'id bin Jubayr (may Allah have mercy on him) said: Abd al-Rahman bin Abza (may Allah be pleased with him) instructed me to ask Sayyiduna Ibn Abbas (may Allah be pleased with him) about these two verses, what they mean. The first is the verse of Surah al-Furqan: 'And those who do not call upon another god along with Allah' up to His words 'nor commit fornication' [Surah al-Furqan: 68], and the second is verse 93 of Surah al-Nisa: 'And whoever kills a believer intentionally' to the end [Surah al-Nisa: 93]. So I asked Sayyiduna Ibn Abbas (may Allah be pleased with him), and he said: When the verse of Surah al-Furqan was revealed, the polytheists of Makkah said: We have killed the soul which Allah forbade, we have called upon another god along with Allah, and we have committed indecencies. So Allah Almighty revealed: 'Except whoever repents and believes' up to His words 'good deeds' [Surah al-Furqan: 70]; so this is for those people. And the verse of Surah al-Nisa is about a person who knows Islam and its laws and then kills (a believer intentionally); there is no repentance for him. I mentioned this to Mujahid (may Allah have mercy on him), and he said: Except the one who regrets.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / 15827