देही नित्यमवध्योऽयं देहे सर्वस्य भारत। तस्मात्सर्वाणि भूतानि न त्वं शोचितुमर्हसि ॥

dehī nityam avadhyo ’yaṁ dehe sarvasya bhārata tasmāt sarvāṇi bhūtāni na tvaṁ śhochitum arhasi

This dweller in the body of everyone is ever invulnerable, O Bhârata; therefore thou shouldst not grieve for any creature.

In plain words: The soul living in every body can never be harmed. So Arjuna should not grieve for any living being.