It is narrated from Sayyiduna Zayd bin Thabit (may Allah be pleased with him); he said: When this verse of Surah al-Furqan was revealed, "And those who do not invoke another god along with Allah, and do not kill the soul that Allah has made sacred except by right" [Surah al-Furqan: 68], we were amazed at its leniency. We remained in that state for seven months, then this verse of Surah al-Nisa was revealed: "And whoever kills a believer intentionally, his recompense is Hell, abiding therein forever, and Allah is angry with him and has cursed him" [Surah al-Nisa: 93], until the verse was complete.
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4850