The Pradhana Satakam is a work by the Sri Vaishnava Acharya Vedanta Desikan describing 100 important things. In verse 7 of the Pradhana Satakam, Vedanta Desikan says that the Pranava is the most important word in the Ashtakshari Mantra, an important Mantra for Sri Vaishnavas:
immantrattin pataṅkaḷ mūṉṟil 'girāmasmi ekam akṣaram', 'praṇavō hyakṣaram param' eṉkiṟa mūlamākiya oṟṟai eḻuttu pradhanam
Among the three words of the Ashtakshari Mantra, the single-lettered Pranava, which is described by the statements “among words I am the the one-lettered word” and “the Pranava is the highest letter”, and which is the root of all things, is the most important.
I’m interested in the scriptural quotes provided. The first one is from Bhagavad Gita verse 10.25:
maharṣīṇāṁ bhṛgur ahaṁ girāmasmi ekam akṣaram| yajñānāṁ japa-yajño’smi sthāvarāṇāṁ himālayaḥ ||
But my question is, what scripture is the second verse quoted, “praṇavō hyakṣaram param”, from? A Google search doesn’t turn up anything.
Do any commentaries on the Pradhana Satakam shed light on this? Note that I want this specific verse, not other verses that talk about the greatness of the Pranava.