Plutarch, Parallela minora
Frank Cole Babbitt, Ed.

Xerxes with five million men anchored near Artemisium and declared war on the inhabitants. The Athenians were in confusion and sent Agesilaüs, the brother of Themistocles, as a spy, although his father Neocles had seen in a dream his son deprived of both his hands. Agesilaüs, arriving among the barbarians in Persian garb, slew Mardonius, one of the king's bodyguards, supposing him to be Xerxes. He was arrested by the bystanders and led in bonds to the king. The aforesaid king was about to offer sacrifice at the altar of the Sun, and Agesilaüs placed his right hand upon the altar ; and when he had endured the cruel torture without a groan, he was freed from his bonds, whereupon he declared, ‘All we Athenians are men of this sort; if you do not believe me, I will place my left hand also on the altar.’ Xerxes was frightened and gave command that he be kept under guard.¹ This Agatharchides the Samian relates in the second book of his Persian History.

Porsenna, king of the Etruscans, made a foray on the other side of the river Tiber and warred against the Romans ; he intercepted their abundant supply [p. 261] of grain and oppressed the aforesaid with famine.² The senate was in confusion ; but Mucius one of the nobles, with the consuls' authorization, took four hundred men of his own age in civilian dress, and crossed the river. He observed one of the tyrant's bodyguards distributing provisions to the officers and, supposing him to be Porsenna, killed him. When he was led to the king, he put forth his right hand into the sacrificial fire ; and dissembling his torments with a stout heart, he said with a smile, ‘Ruffian, I am free, whether you will or no. Know that there are against you even now in your camp four hundred of us that seek to slay you.’ Porsenna was frightened, and made a truce with the Romans.³ This Aristeides the Milesian relates in the third book of his Histories.