There is a passage of Plutarch's tract De Profectibus in Virtute 7 in which Sophocles discusses his own growth as a writer.
A likely source of this material for Plutarch was the Epidemiae of Ion of Chios, a book that recorded many conversations of Sophocles. This book is a likely candidate to have contained Sophocles' discourse on his own development because Ion was a friend of Sophocles, and the book is known to have been used by Plutarch. Though some interpretations of Plutarch's words suggest that Sophocles says that he imitated Aeschylus, the translation does not fit grammatically, nor does the interpretation that Sophocles said that he was making fun of Aeschylus' works. C. M. Bowra argues for the following translation of the line: "After practising to the full the bigness of Aeschylus, then the painful ingenuity of my own invention, now in the third stage I am changing to the kind of diction which is most expressive of character and best."
Here Sophocles says that he has completed a stage of Aeschylus' work, meaning that he went through a phase of imitating Aeschylus' style but is finished with that. Sophocles' opinion of Aeschylus was mixed. He certainly respected him enough to imitate his work early on in his career, but he had reservations about Aeschylus' style, and thus did not keep his imitation up. Sophocles' first stage, in which he imitated Aeschylus, is marked by "Aeschylean pomp in the language". Sophocles' second stage was entirely his own. He introduced new ways of evoking feeling out of an audience, like in his Ajax when he is mocked by Athene, then the stage is emptied so that he may commit suicide alone. Sophocles mentions a third stage, distinct from the other two, in his discussion of his development. The third stage pays more heed to diction. His characters spoke in a way that was more natural to them and more expressive of their individual character feelings.