Parmarthika Sathya is implicitly mentioned in Mandukya Upanishad verse 7:
नान्तःप्रज्ञं न बहिःप्रज्ञं नोभयतःप्रज्ञं न प्रज्ञानघनं न प्रज्ञं
प्रपञ्चोपशमं शान्तं शिवमद्वैतं चतुर्थं मन्यन्ते स आत्मा स विज्ञेयः ||
Turīya is not that which is conscious of the internal (subjective)
world, nor that which is conscious of the external (objective) world,
nor that which is conscious of both, nor that which is a mass all
sentiency, nor that which is simple consciousness, nor that which is
insentient. (It is) unseen (by any sense organ), not related to
anything, incomprehensible (by the mind), un-infer-able, unthinkable,
indescribable, essentially of the nature of Consciousness constituting
the Self alone, negation of all phenomena, the Peaceful, all Bliss and
the Non-dual. This is what is known as the fourth (Turīya). This is
the Ātman and it has to be realized.
The first word which is written in bold letters is Avyavaharyam, which means opposite of Vyavaharika reality, i.e., that which is not conventional reality. This verse is saying Turiya is something which can never come into conventional reality and thus it is Avyavaharika. The second bold word is Advaitam which implies Turiya is Non-dual.