Bhagavad Gita: Chapter 2, Verse 30
देही नित्यमवध्योऽयं देहे सर्वस्य भारत |
तस्मात्सर्वाणि भूतानि न त्वं शोचितुमर्हसि || 30||
dehī nityam avadhyo ’yaṁ dehe sarvasya bhārata
tasmāt sarvāṇi bhūtāni na tvaṁ śhochitum arhasi
O Arjun, the soul that dwells within the body is immortal; therefore, you should not mourn for anyone.
Often, in the course of his teachings, Shree Krishna explains a concept in a few verses, and then states a verse summarizing those teachings. This verse is a summary of the instructions on the immortality of the self, and its distinction from the body.