I believe that St Thomas had no knowledge of Plato at first hand, not even in a Latin translation. He knew him only through the citations of Aristotle, and commentators, mostly Neoplatonists. For the opinion here ascribed to Plato, see Plato's Phaedo, pp. 80, 94; Phaedrus, 245, 246; Laws, 896, 897. It appears not so much explicitly in any one passage, as implicitly in the general tenor of Plato's philosophy, especially in the strong opposition, and even repugnance, which he supposes to obtain between soul and body; in his doctrine of the pre-existence of soul before body, also of the transmigration of souls (which argues a very loose connection between the soul and the particular body which it inhabits): likewise in this general difference between Aristotelian and Platonic 'forms,' that while Aristotle's 'forms' inhere in sensible things, Plato's 'forms,' or eide, stand apart; so that even though Plato had allowed the soul to be the 'form' of the body, which he did not allow, still even so he would have kept this 'form' apart from and independent of the body. Plato in fact detested material substance, and would not have spirit bound up with matter. Spirit was to rule matter; and when for its punishment it got entangled in matter, as in man, and still more in the lower animals, it was to do its best to break away, and (in man) to live a life of its own, as much apart from the body and bodily senses as possible.
Of God and His Creatures: 2.57