From Ovid's Metamorphoses, Book 9:
Hercules met the Centaur Nessus on the way back home with his wife, Deianira. Nessus lusts for Deianira and attempts to take her by offering to help take her across a swollen river. Hercules shoots Nessus with a arrow dipped in Hydra's blood. As he dies, Nessus gives his (blood covered) garment to Deianara, telling her it is a talisman that would renew love:
His garment, in the reeking purple dyed,
To rouse love's passion, he presents the bride.

When Deianara "a long interval of time" later comes to believe that Hercules had fallen for Iole, she sends the tunic to Hercules, who wears it and falls victim to Hydra poison still within it. Lychas, who bore the garment to Hercules, is turned into to a rock for his trouble.

Original text, as translated under Sir Samuel Garth, available at