Bhagavad Gita 2:11: The Blessed One said: While speaking learned words, you are mourning for what is not worthy of grief. Those who are wise lament neither for the living nor for the dead.
2:12: Never was there a time when I did not exist, nor you, nor all these kings, nor in the future shall any of us cease to be.
2:13: As the embodied soul continuously passes, in this body, from boyhood to youth to old age, the soul also passes into another body at death. A sober person is not bewildered by such a change.
2:14: O son of Kunti, sensory perceptions afflict one with cold, heat, pleasure and pain. They appear and disappear and are impermanent. Therefore endure them all O descendant of Bharata.
2:15: One who is never distressed, O best among men, and who remains unaltered [in the face of] suffering and pleasure and is patient, such a person is eligible for immortality.
2:16: Being does not come from the non-existent, nor does non-being arise from the eternal. This is the considered conclusion of those who see the truth.
(my translation).