I think that it is a challenge to find exact quotes from myths because the translation and recitation of myths always changes. It is this human quality to them where their thematic application becomes universal. Despite the temporal changes they experience, the themes do not perish. For the narrative of Perseus, I would suggest that one thematic statement would come from the prophecy that Perseus would kill his grandfather, King Acrisius of Argos. This is significant because it shows the role of fate in the Greek mythology that even a hero like Perseus must endure and how one, despite all of their accomplishments, has to bow to the power of fate.
At the same time, I would suggest that the thematic statement revealed by Perseus' killing of Medusa is reflective of much in way of significance. The heroic deed is seen with the slaying of Medusa. Yet, I would suggest that another level of thematic importance to this would be in the translated statement of how she is killed. The use of bronze shield as a mirror is thematically important. Ugliness, in the form of Medusa, is shown to be an internal quality. When Medusa sees her own hideous countenance, she dies. The Greeks might be suggesting that ugliness dies when it is confronted internally. When individuals confront openly and honestly their own ugliness, it dies because it arises from the individual, in question. Medusa never had to confront her own ugliness and because of that, she clung to it as a strength. When she confronts her own ugliness, it dies out. For the Greeks, the notion of purity both internally and externally is part of the conception of self that is reaffirmed through the Perseus myth, making this an important quotation or thematic message from it.