In Godwin's novel called Caleb Williams. He commits murder, and keeps a narrative of the transaction in an iron chest. Williams, a lad in his employ, opens the chest, and is caught in the act by Falkland. The lad runs away, but is hunted down. This tale, dramatised by Colman, is entitled The Iron Chest.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894