Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) said: "Whoever takes part in shedding unlawful blood with even half a word will come on the Day of Resurrection with written between his eyes: Despairing of the mercy of Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11056