It is narrated from Sayyiduna Zayd bin Thabit (may Allah be pleased with him) that he said: "The verse in Surah al-Nisa that is severe about killing a soul was revealed six months after this verse of Surah al-Furqan: 'And whoever does that will meet the penalty of sin' [Surah al-Furqan: 68]."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4887