Hadith.5237 - It was reported to the Commander of the Faithful (as) about three men: one of them restrained a man, another approached and killed him, and the third stood by watching them.
Imam Ali ibn Abi Talib (as) ruled that the one who merely watched should have his eyes gouged out. Imam (as) ruled that the one who restrained the victim should be imprisoned until he dies, just as he held the victim until death. As for the one who committed the murder, Imam (as) ruled that he should be executed.