Isma'il bin Umayyah said: The Messenger of Allah (peace and blessings be upon him) gave a judgement concerning a man whom one person held so that another could kill him, and he was killed: "The killer is to be killed and the one who held him is to be imprisoned."
A similar judgement is also reported from Ali (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16030