Sunan an-Nasa'i 3989
سنن النسائي#3989HasanIt 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 Ghuddah-
Zubair Ali ZaiHasan
In-book reference: 37:24