Sunan an-Nasa'i, Volume 5, Book 37, Hadith 4000 (DarusSalam) ⓘ
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.'"
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 " .
also referenced under: Hadith 3995 (Reference) & Book 37, Hadith 30 (In-Book)