Plutarch, Theseus
Bernadotte Perrin, Ed.

The friendship of Peirithous and Theseus is said to have come about in the following manner. Theseus had a very great reputation for strength and bravery, and Peirithous was desirous of making test and proof of it. Accordingly, he drove Theseus's cattle away from Marathon, and when he learned that their owner was pursuing him in arms, he did not fly, but turned back and met him. [2] When, however, each beheld the other with astonishment at his beauty and admiration of his daring, they refrained from battle, and Peirithous, stretching out his hand the first, bade Theseus himself be judge of his robbery, for he would willingly submit to any penalty which the other might assign. Then Theseus not only remitted his penalty, but invited him to be a friend and brother in arms; whereupon they ratified their friendship with oaths. [3]

After this, when Peirithous was about to marry Deidameia, he asked Theseus to come to the wedding, and see the country, and become acquainted with the Iapithae. Now he had invited the Centaurs also to the wedding feast. And when these were flown with insolence and wine, and laid hands upon the women, the Lapithae took vengeance upon them. Some of them they slew upon the spot, the rest they afterwards overcame in war and expelled from the country, Theseus fighting with them at the banquet and in the war. [4] Herodorus, however, says that this was not how it happened, but that the war was already in progress when Theseus came to the aid of the Lapithae and that on his way thither he had his first sight of Heracles, having made it his business to seek him out at Trachis, where the hero was already resting from his wandering and labours; and he says the interview passed with mutual expressions of honor, friendliness, and generous praise. [5] Notwithstanding, one might better side with those historians who say that the heroes had frequent interviews with one another, and that it was at the instigation of Theseus that Heracles was initiated into the mysteries at Eleusis, and purified before his initiation, when he requested it on account of sundry rash acts.