'Abdullah Ibn Mas'ud (RAA) narrated that The Messenger of Allah (ﷺ) said:
"The first issues to be judged among people on the Day of Resurrection, are those of unlawful blood-shed." Agreed upon. (FR)
English reference (FR): Book 9, Hadith 1201 (FR)
Arabic reference (FR): Book 9, Hadith 1171 (FR)