Cited in the foreword to 'Panchapadika Vivaranam with two commentaries' by Polagam Rama Sastri: (Published by GOML, 1958, Madras):
He cites this verse, in turn cited by Nagesha Bhatta, as from the Brihannaaradiya:
नासद्रूपा न सद्रूपा माया नैवोभयात्मिका । अनिर्वाच्या ततो ज्ञेया मिथ्याभूता सनातनी ||
nAsadrUpA na sadrUpa naivobhayAtmikA. anirvAchyA tato jneyA mithyAbhUtA sanAtanI.
Maya is neither sat, nor asat, nor sat-asat. It is anirvAchyA and hence to be known as mithya and anAdi.
This tallies with the Shankara bhashyam: BSB 2.1.14:
सर्वज्ञस्येश्वरस्यात्मभूते इवाविद्याकल्पिते नामरूपे तत्त्वान्यत्वाभ्यामनिर्वचनीये संसारप्रपञ्चबीजभूते सर्वज्ञस्येश्वरस्य मायाशक्तिः प्रकृतिरिति च श्रुतिस्मृत्योरभिलप्येते ;
This verse is found in Sourapurana
सदसद्भ्यामन्यरूपा मिथ्याभूता सनातना
warm regards
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