Matsya Purana — Manvantaras
शिष्टा यस्माच्चरन्त्येनं मनुः सप्तर्षयश्च ह मन्वन्तरेषु सर्वेषु शिष्टाचारस्ततः स्मृतः //
śiṣṭā yasmāccarantyenaṃ manuḥ saptarṣayaśca ha manvantareṣu sarveṣu śiṣṭācārastataḥ smṛtaḥ //
Because cultured and exemplary people practice this, and because Manu and the Seven Sages indeed follow it in every Manvantara, it is therefore remembered as śiṣṭācāra—authoritative customary conduct.
It does not describe Pralaya directly; it emphasizes continuity of dharma across cosmic cycles (Manvantaras), implying that righteous norms persist even as ages change.
It defines a practical standard for duty: rulers and householders should model their conduct on śiṣṭācāra—what exemplary, learned people uphold—validated by Manu and the Saptarishis as a reliable guide when moral questions arise.
No specific Vāstu or ritual procedure is stated, but the principle supports Vāstu/ritual manuals in practice: correct temple-building or rite-performance is confirmed by śiṣṭācāra—established expert tradition—alongside scriptural injunctions.