P. 294 (with art)

 Paris, Louvre  G341: Attic Kylix Krater by the Niobid Painter, Herakles with heroes among them possibly  Theseus.

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Isokrates, Helen 20:

So grateful was Theseus to Peirithos, his partner in the abduction, that when Peirithos wished to woo Persephon, the daughter of Zeus and Demeter, and summoned him to the descent into Hades to obtain her, when Theseus found that he could not by his warnings dissuade his friend, although the danger was manifest he nevertheless accompanied him, for he was of opinion that he owed this debt of gratitude—to decline no task enjoined by Peirithos in return for his help in his own perilous enterprise (original Greek).

Diodoros Siculus, Library of History 4.63.4-5:

Peirithoüs now decided to seek the hand of Persephonê in marriage, and when he asked Theseus to make the journey with him Theseus at first endeavoured to dissuade him and to turn him away from such a deed as being impious; but since Peirithoüs firmly insisted upon it Theseus was bound by the oaths to join with him in the deed. And when they had at last made their way below to the regions of Hades, it came to pass that because of the impiety of their act they were both put in chains, and although Theseus was later let go by reason of the favour with which Heracles regarded him, Peirithoüs because of the impiety remained in Hades, enduring everlasting punishment; but some writers of myths say that both of them never returned (original Greek).

 Edited by Aaron J. Ivey, Graduate Teaching Assistant, Department of Classics, University of Georgia, June 2016; by R. Ross Holloway, Elisha Benjamin Andrews Professor Emeritus, Joukowsky Institute for Archaeology and the Ancient World, Brown Univ., June 2016; and by Frances Van Keuren, Prof. Emerita, Lamar Dodd School of Art, Univ. of Georgia, July 2016.