Bhagavad Gita: Chapter 2, Verse 28

अव्यक्तादीनि भूतानि व्यक्तमध्यानि भारत ।
अव्यक्तनिधनान्येव तत्र का परिदेवना ॥ २-२८ ॥

avyaktādīni bhūtāni vyakta-madhyāni bhārata
avyakta-nidhanāny eva tatra kā paridevanā 2.28

O scion of Bharat, all created beings are unmanifest in their beginning, manifest in their interim state, and unmanifest again when annihilated. So why grieve?