Matsya Purana — Manvantaras
शिष्टैराचर्यते यस्मात् पुनश्चैव मनुक्षये पूर्वैःपूर्वैर्मतत्वाच्च शिष्टाचारः स शाश्वतः //
śiṣṭairācaryate yasmāt punaścaiva manukṣaye pūrvaiḥpūrvairmatatvācca śiṣṭācāraḥ sa śāśvataḥ //
Because it is practiced by the śiṣṭas, and because—even when a Manu’s age comes to an end—it is upheld again; and because it is approved by the ancients, generation after generation, that śiṣṭācāra, the established conduct of the learned, is eternal.
It does not describe Pralaya directly; it implies continuity across cosmic cycles by stating that śiṣṭācāra (the conduct of the learned) persists and is reaffirmed even when a Manu’s period ends.
It grounds dharma in living standards: rulers and householders should follow the time-tested practices of exemplary, learned people, since such conduct is treated as authoritative across generations and Manvantaras.
No specific Vāstu or ritual rule is stated; the principle is methodological—ritual and temple-building traditions gain authority when they are consistently upheld by śiṣṭas and validated by long-standing lineage practice.