Chapter 2 · The Yoga of Knowledge · Krishna said
Bhagavad Gita 2.30 — The soul cannot be slain
देही नित्यमवध्योऽयं देहे सर्वस्य भारत। तस्मात्सर्वाणि भूतानि न त्वं शोचितुमर्हसि ॥
dehī nityamavadhyo'yaṃ dehe sarvasya bhārata | tasmātsarvāṇi bhūtāni na tvaṃ śocitumarhasi ||
Meaning
The soul living in every body can never be killed, Arjuna. So you should not grieve for any being.