Abdullah bin Mas'ud (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "Whoever is killed unjustly in the world, a share of the sin of his killing also goes to the first son of Adam (peace be upon him), because he was the first to initiate killing."
In the narration of Abu Mu'awiyah the words are: "No one is killed unjustly in the world except that a share of its sin falls upon the son of Adam, because it was he who initiated killing."
Bukhari recorded it from al-Humaydi, and Muslim from Ibn Abi Umar from Sufyan. Allah Almighty says: 'And whoever kills a believer intentionally, his recompense is Hell, abiding therein forever, and Allah is angry with him, has cursed him and has prepared for him a great punishment.' [Surah al-Nisa: 93]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / 15824