Apollodorus Library 2.7.6

[6]
  And Hercules marched with the Calydonians against the Thesprotians, and having taken the city of Ephyra, of which Phylas was king, he had intercourse with the king's daughter Astyoche, and became the father of Tlepolemus. ¹ While he stayed among them, he sent word to Thespius to keep seven of his sons, to send three to Thebes and to despatch the remaining forty to the island of Sardinia to plant a colony. ² After these events, as he was feasting with Oeneus, he killed with a blow of his knuckles endeavored, son of Architeles, when the lad was pouring water on his hands; now the lad was a kinsman of Oeneus. ³ Seeing that it was an accident, the lad's father pardoned Hercules; but Hercules wished, in accordance with the law, to suffer the penalty of exile, and resolved to depart to Ceyx at Trachis. And taking Deianira with him, he came to the river Evenus, at which the centaur Nessus sat and ferried passengers across for hire, ⁴ alleging that he had received the ferry from the gods for his righteousness. So Hercules crossed the river by himself, but on being asked to pay the fare he entrusted Deianira to Nessus to carry over. But he, in ferrying her across, attempted to violate her. She cried out, Hercules heard her, and shot Nessus to the heart when he emerged from the river. Being at the point of death, Nessus called Deianira to him and said that if she would have a love charm to operate on Hercules she should mix the seed he had dropped on the ground with the blood that flowed from the wound inflicted by the barb. She did so and kept it by her.