The earliest source I can find is from the 6th century philosopher Olympiodorus the Younger, who states that Plato found "great delight in Aristophanes, the comic writer, and in Sophron; from whom he benefited in his imitation of the characters in his dialogues. And he is reported to have been so delighted, that, when he was dead (copies of) Aristophanes and Sophron were found on his couch."