Matsya Purana — Catalogue of the Eighteen Puranas
प्रवृत्तिः सर्वशास्त्राणां पुराणस्याभवत्ततः कालेनाग्रहणं दृष्ट्वा पुराणस्य ततो नृप //
pravṛttiḥ sarvaśāstrāṇāṃ purāṇasyābhavattataḥ kālenāgrahaṇaṃ dṛṣṭvā purāṇasya tato nṛpa //
From that Purāṇa arose the onward course of all the śāstras. Then, O King, seeing that in the passage of time the Purāṇa was no longer being properly retained (studied and preserved), the tradition-bearer acted accordingly.
It does not describe Pralaya directly; it highlights how sacred knowledge (the Purāṇa) underpins later śāstras and how, over time, neglect or loss of retention can occur—an intellectual decline that often accompanies cyclical ages.
By addressing “O King,” it implies a ruler’s dharma includes supporting the preservation and teaching of foundational texts—patronage of recitation, learning, and safeguarding of tradition so that society’s śāstric order remains intact.
No specific Vāstu or ritual rule is stated; the verse provides the meta-principle that Purāṇic knowledge is a source-stream for later technical disciplines (including ritual manuals and Vāstu-śāstra), so preserving the Purāṇa supports those applied sciences indirectly.