Matsya Purana — Manvantaras
दृष्टानुभूतमर्थं च यः पृष्टो न विगूहते यथाभूतप्रवादस्तु इत्येतत्सत्यलक्षणम् //
dṛṣṭānubhūtamarthaṃ ca yaḥ pṛṣṭo na vigūhate yathābhūtapravādastu ityetatsatyalakṣaṇam //
When questioned, one who does not conceal a matter that has been seen or personally experienced—who states things as they truly are—this is the defining mark of truthfulness.
It does not address pralaya directly; it defines satya (truth) as reporting what one has truly seen or experienced without concealment.
It frames truthful testimony as a dharmic duty: a king (in judgment) and a householder (in daily dealings) should not hide known facts when asked, and should speak in accordance with reality (yathābhūta).
No Vāstu or ritual procedure is mentioned; the verse is an ethical rule about truthful speech, relevant indirectly to ritual integrity (truthful vows and declarations).