It is narrated from al-Zuhri (may Allah have mercy on him) as marfu': "Whoever helps in the killing of a believer even with half a word, on the Day of Resurrection there will be written between his eyes: 'Ayisun min rahmatillah' (Deprived of the mercy of Allah)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15868