مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4096
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah said: The Messenger of Allah (ﷺ) said: “No soul is killed unlawfully, but there is a share of the sin on the first son of Adam, because he was the first one to set the precedent of killing.”
Musnad Ahmad 4123Its isnad is saheeh, Muslim (6867)]