Bhagavad Gita Chapter 8, Verse 12

अव्यक्ताद्व्यक्तय: सर्वा: प्रभवन्त्यहरागमे | रात्र्यागमे प्रलीयन्ते तत्रैवाव्यक्तसंज्ञके ||

avyaktād vyaktayaḥ sarvāḥ prabhavanti aha-rāgame rātry-āgame pralīyante tatraiva avyakta-saṁjñake

All manifest beings arise from the unmanifest at the approach of day, and at the approach of night they dissolve back into that which is called the unmanifest.

Krishna describes the cycle of manifestation: 'Avyaktad vyaktayah sarvah prabhavanti aharagame'—all (sarvah) manifest beings (vyaktayah) arise (prabhavanti) from the unmanifest (avyaktat) at the approach (agame) of day (ahar). 'Ratryagame pralayante tatraiva avyakta-samjnake'—at the approach (agame) of night (ratri) they dissolve (pralayante) back into that (tatra eva) which is called (samjnake) the unmanifest (avyakta). This verse describes the cosmic cycle: all manifest beings come from the unmanifest and return to it. Day represents manifestation—beings come forth. Night represents dissolution—beings merge back. This is the cycle of creation and destruction. But the unmanifest itself is eternal—it's the foundation that never changes. This helps you understand: everything manifest is temporary, but the unmanifest (Brahman) is eternal. This is what you're preparing to return to at death—not the temporary manifest, but the eternal unmanifest.