The Book of Fighting [The Prohibition of Bloodshed]
37/2. Chapter:
The Gravity of the Sin of Shedding Blood
Sunan an-Nasai
أَخْبَرَنَا أَحْمَدُ بْنُ سُلَيْمَانَ، قَالَ حَدَّثَنَا أَبُو دَاوُدَ، عَنْ سُفْيَانَ، عَنِ الأَعْمَشِ، عَنْ أَبِي وَائِلٍ، قَالَ قَالَ عَبْدُ اللَّهِ أَوَّلُ مَا يُقْضَى بَيْنَ النَّاسِ يَوْمَ الْقِيَامَةِ فِي الدِّمَاءِ .
It was narrated from 'Abdullah that: The Messenger of Allah [SAW] said: "The first matter concerning which scores will be settled among the people on the Day of Resurrection will be bloodshed."
Reference : Sunan an-Nasa'i 3993In-book reference : Book 37, Hadith 28English translation : Vol. 5, Book 37, Hadith 3998