Sunan an Nasai
4006
Hasan Sahih+2

It was narrated that Zaid bin Thabit said:"This Verse - 'And whoever kills a believer intentionally, his recompense is Hell' - was revealed six months after the Verse which was revealed in Surat Al-Furqan

Hasan SahihAbu Ghuddah
Hasan SahihAl-Albani