عَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ رَضِيَ اللَّهُ عَنْهُ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ ((لَا تُقْتَلُ نَفْسٌ ظُلْمًا إِلَّا كَانَ عَلَى ابْنِ آدَمَ الْأَوَّلِ كِفْلٌ مِنْ دَمِهَا لِأَنَّهُ كَانَ أَوَّلَ مَنْ سَنَّ الْقَتْلَ))
It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: For every soul that is killed unjustly, the first son of Adam (peace be upon him) receives a share of the sin of that blood, because he was the first person who started killing.