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 حَرْبٍ، قَالَ حَدَّثَنَا أَبُو مُعَاوِيَةَ، عَنِ الأَعْمَشِ، عَنْ أَبِي وَائِلٍ، عَنْ عَمْرِو بْنِ شُرَحْبِيلَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " أَوَّلُ مَا يُقْضَى فِيهِ بَيْنَ النَّاسِ يَوْمَ الْقِيَامَةِ فِي الدِّمَاءِ " .
It was narrated that 'Amr bin Shurahbil said: "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 3995In-book reference : Book 37, Hadith 30English translation : Vol. 5, Book 37, Hadith 4000