Plutarch, De E apud Delphos
Goodwin, Ed.

Ammonius having spoken thus, Lamprias the Delphian said: The reason indeed which we have heard of this is plain and very short; for they say that those Sages, who were by some called Sophisters, were but five, Chilo, Thales, Solon, Bias, and Pittacus. But after that Cleobulus the tyrant of the Lindians, and Periander the Corinthian, though wholly destitute of virtue and wisdom, had by their power, friends, and courtesy forced a reputation, they usurped the name of Sages, and set forth and dispersed all over Greece certain sentences and sayings, not unlike to those which had been spoken by the five former wise men. The five, however, being discontented at this, would not reprove their arrogancy, nor openly contest and enter into quarrels for glory with men of so great power; but assembling here together, and consulting with one another, [p. 481] they consecrated the letter E, which is in the order of the alphabet the fifth, and signifies five in number, protesting of themselves before the God that they were but five, and rejecting and abdicating the sixth and seventh as not belonging to them. Now that these things are not spoken beside the cushion, any one might understand who should have heard those who have care of the temple naming the golden EI the EI of Livia, the wife of Augustus Caesar; and the brazen one the El of the Athenians; but the first and ancientest of all, which is the wooden one, they call the EI of the Sages, as not being of any one, but the common dedication of them all.