The Messenger of Allah ﷺ said, "No soul is killed unjustly without the first son of Adam bearing a share of the guilt for its blood, because he was the first to establish the practice of killing."
حدثنا أبو معاوية حدثنا الأعمش عن عبد الله بن مرة عن مسروق
قال رسول الله ﷺ لا تقتل نفس ظلما إلا كان على ابن آدم الأول كفل من دمها لأنه كان أول من سن القتل
Its isnad is sahih. Al-Bukhari and Muslim narrated it, as in al-Muntaqa 3959. Tirmidhi, Nasai, and Ibn Majah also narrated it, as in al-Dhakhair 4969. Kifl, with kaf vowelled with kasra and fa with sukun, means share or portion.