the emperor, condemned a knight to death because the companion who went out with him returned not. “Thou hast slain thy fellow,” said the emperor, “and must die. Go,” continued he, to another knight, “and lead him to death.” On their way they met the knight supposed to be dead, and returned to Eraclius, who, instead of revoking his sentence, ordered all three to be put to death—the first because he had already condemned him to death; the second because he had disobeyed his orders; and the third because he was the real cause of the death of the other two. Chaucer tells this anecdote in his Sompnoures Tale. It is told of Cornelius Piso by Seneca in his De Ira, lib. i. 16; but in the Gesta Romanorum it is ascribed to Eraclius.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894