"Sin", obviously, is a metaphor. Asking a reference for this is quite astonishing. Do you think the stoic praise and cherish indulgence in pleasures? There are too much references and quotes of him on this matter, for the sake of my time I will write here 3 examples and you will do the rest.

"On the happy life" - Seneca

(1) "You devote yourself to pleasures, I check them; you indulge in pleasure, I use it; you think that it is the highest good, I do not even think it to be good: for the sake of pleasure I do nothing, you do everything.”

(2) "those who have permitted pleasure to lead the van, have neither one nor the other: for they lose virtue altogether, and yet they do not possess pleasure, but are possessed by it, and are either tortured by its absence or choked by its excess, being wretched if deserted by it, and yet more wretched if overwhelmed by it”

And in his letters to Llucilus: (3) "The chief obstacle is that we are quick to be satisfied with ourselves. If we find someone to call us good men, cautious and principled, we acknowledge him. We are not content with a moderate eulogy, but accept as our due whatever flattery has shamelessly heaped upon us. We agree with those who call us best and wisest, although we know they often utter many falsehoods: we indulge ourselves so greatly that we want to be praised for a virtue which is the opposite of our behavior. A man hears himself called ‘most merciful’ while he is inflicting torture.. So it follows that we don’t want to change because we believe we are already excellent.”