Metamorphoses Book 9: Hercules, Nessus, and Deianira
When Hercules was taking his wife to his home in Tiryns, they came to a swollen river. Nessus, a centaur, offered to help Hercules get Deianira across the river. Hercules swam across and heard Deianira yelling for him. Assuming that the centaur was kidnapping her to rape her, Hercules shot him with an arrow tipped with Hydra's poison. The centaur, seeking vengeance, gave his shirt covered in Hydra's poison to Deianira and told her that it was a talisman that would renew love.
Years later, when Hercules had completed his labors and was still far from home, Deianira heard from "Rumour who talks and loves to tangle true / With false, and from near nothing flourishes / On her own lies" Book 9 -- Hercules, Nessus, and Deianira, line 141-3 that his heart had been captured by another woman. She sent him the shirt not realizing that it was poisoned. Hercules, unaware of its taint, wore the shirt.