عَنْ عَبْدِ اللَّهِ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ: ((لَا تُقْتَلُ نَفْسٌ ظُلْمًا إِلَّا كَانَ عَلَى ابْنِ آدَمَ الْأَوَّلِ كِفْلٌ مِنْ دِمِهَا لِأَنَّهُ كَانَ أَوَّلَ مَنْ سَنَّ الْقَتْلَ))
Narrated by Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him): The Messenger of Allah (peace and blessings be upon him) said: Whenever a soul is killed unjustly, a share of its unjust blood is also upon Adam's first son, because he was the first person who initiated the practice of unjust killing.