Sunan an-Nasa'i, Volume 5, Book 37, Hadith 3997 (DarusSalam) ⓘ
It was narrated that 'Abdullah said: "The Messenger of Allah [SAW] said: 'The first matter concerning which judgment will be passed among the people will be bloodshed.'"
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، عَنْ خَالِدٍ، حَدَّثَنَا شُعْبَةُ، عَنْ سُلَيْمَانَ، قَالَ سَمِعْتُ أَبَا وَائِلٍ، يُحَدِّثُ عَنْ عَبْدِ اللَّهِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " أَوَّلُ مَا يُحْكَمُ بَيْنَ النَّاسِ فِي الدِّمَاءِ " .
also referenced under: Hadith 3992 (Reference) & Book 37, Hadith 27 (In-Book)