Sunan an-Nasa'i 3988
سنن النسائي#3988SahihIt was narrated that 'Abdullah bin 'Amr said:"Killing a believer is more grievous before Allah than the extinction of the whole world
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih Muquf
Al-AlbaniSahih Muquf
Zubair Ali ZaiHasan
In-book reference: 37:23