"A clay tablet reveals the case, in 1850BC, of the murder of a temple employee by three men. The victim's wife knew of the murder but remained silent. Eventually, the crime came to light and the men and woman were charged with murder. Nine witnesses testified against the men and woman and asked for the death penalty for all four. But the wife had two witnesses which told the court that she had been abused by her husband, that she was not part of the murder and that she was even worse off after her husband's death. The men were executed in front of the victim's house but the woman was spared."

http://www.duhaime.org/LawMuseum/LawArticle-44/Duhaimes-Timetable-of-World-Legal-History.aspx

The oldest I know of that is on record