Bhagavad Gita: Chapter 2, Verse 27

जातस्य हि ध्रुवो मृत्युर्ध्रुवं जन्म मृतस्य च ।
तस्मादपरिहार्येऽर्थे न त्वं शोचितुमर्हसि ॥ २-२७ ॥

jātasya hi dhruvo mṛtyur dhruvaṁ janma mṛtasya ca
tasmād aparihārye ’rthe na tvaṁ śocitum arhasi 2.27

One who has taken his birth is sure to die, and rebirth is sure for one who has died. Therefore, in the unavoidable discharge of your duty, you should not lament.