Matsya Purana — Narasimha’s Victory over Hiraṇyakaśipu and the Catalogue of Apocalyptic Omens
न च तं चालयामासुर् दैत्यौघा देवसत्तमम् भीमवेगो ऽचलश्रेष्ठं समुद्र इव मन्दरम् //
na ca taṃ cālayāmāsur daityaughā devasattamam bhīmavego 'calaśreṣṭhaṃ samudra iva mandaram //
Yet the rushing hosts of Daityas could not make that best of the gods so much as stir—steadfast and supreme like the loftiest mountain, as the ocean (cannot dislodge) Mandara.
It does not describe Pralaya directly; it uses the ocean–Mandara comparison as a poetic image of immovability and overwhelming force, not as a flood/dissolution episode.
By praising unwavering steadiness under assault, it supports the Purāṇic ethic of dhairya (fortitude): a king (or householder) should remain unshaken in dharma despite pressure from hostile forces or crises.
No direct Vāstu or ritual rule is stated; the verse’s key takeaway is the symbolic ideal of stability (acalavat)—a quality later valued in temple/axis symbolism, though here it remains a battle simile.