Abu Hurayrah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "Whoever helps, even to the slightest degree, in the killing of a believer will come on the Day of Resurrection in such a state that written on his forehead will be: Deprived of the mercy of Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15865