The Book of Fighting [The Prohibition of Bloodshed]
37/2. Chapter:
The Gravity of the Sin of Shedding Blood
Sunan an-Nasai
أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا مُحَمَّدٌ، عَنْ شُعْبَةَ، عَنْ يَعْلَى، عَنْ أَبِيهِ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، قَالَ قَتْلُ الْمُؤْمِنِ أَعْظَمُ عِنْدَ اللَّهِ مِنْ زَوَالِ الدُّنْيَا .
It was narrated that 'Abdullah bin 'Amr said: "Killing a believer is more grievous before Allah than the extinction of the whole world."
Reference : Sunan an-Nasa'i 3988In-book reference : Book 37, Hadith 23English translation : Vol. 5, Book 37, Hadith 3993