Matsya Purana — Soma
तदा स गच्छति द्रष्टुं दिवाकरनिशाकरौ अमावास्याममावास्यां मातामहपितामहौ //
tadā sa gacchati draṣṭuṃ divākaraniśākarau amāvāsyāmamāvāsyāṃ mātāmahapitāmahau //
Then he goes to behold the Sun and the Moon; and on the day of Amāvāsyā (the New Moon), he reverently beholds—i.e., pays homage to—the maternal grandfather and the paternal grandfather.
This verse is not about Pralaya; it focuses on ritual timekeeping—especially Amāvāsyā—as an auspicious occasion for honoring ancestral lineages.
It frames a dharmic routine: a disciplined person (notably a gṛhastha, and by extension a ruler as moral exemplar) should observe the celestial markers (Sun and Moon) and perform reverence toward elders/ancestors—specifically both maternal and paternal grandfathers—on Amāvāsyā.
The ritual significance is calendrical: Amāvāsyā is highlighted as a key tithi for Pitṛ-related observances (such as śrāddha/tarpaṇa), linking correct timing with correct ancestral veneration.