It was narrated that Mujalid bin 'Awf said: "I heard Kharijah bin Zaid bin Thabit narrate that his father said: (The Verse) 'And whoever kills a believer intentionally, his recompense is Hell' was revealed and we became worried about it. Then the Verse in Al-Furqan 'And those who invoke not any other ilah (god) along with Allah, or kill such person as Allah has forbidden, except by right.' was revealed."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، عَنْ مُسْلِمِ بْنِ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ إِسْحَاقَ، عَنْ أَبِي الزِّنَادِ، عَنْ مُجَالِدِ بْنِ عَوْفٍ، قَالَ سَمِعْتُ خَارِجَةَ بْنَ زَيْدِ بْنِ ثَابِتٍ، يُحَدِّثُ عَنْ أَبِيهِ، أَنَّهُ قَالَ نَزَلَتْ { وَمَنْ يَقْتُلْ مُؤْمِنًا مُتَعَمِّدًا فَجَزَاؤُهُ جَهَنَّمُ خَالِدًا فِيهَا } أَشْفَقْنَا مِنْهَا فَنَزَلَتِ الآيَةُ الَّتِي فِي الْفُرْقَانِ { وَالَّذِينَ لاَ يَدْعُونَ مَعَ اللَّهِ إِلَهًا آخَرَ وَلاَ يَقْتُلُونَ النَّفْسَ الَّتِي حَرَّمَ اللَّهُ إِلاَّ بِالْحَقِّ } .
sunnah.com