Plato's Phaedo concludes the drama of Socrates' final hours wherein he discusses with intimate friends the highest mysteries of living and dying, just before drinking the hemlock.
This is a treasure trove of platonic thought. Among the more intriguing claims is that we ought to believe in the immorality of the soul -if for no other reason because we will lead a better life. Indeed, it might be that we already take our soul to be immortal insofar as we lead moral lives at all.
This dialogue is pretty long so remember to allow enough time.
This audio is well-read: https://librivox.org/phaedo-by-plato/