Bhagavadgita 2.11-20

The Blessed Lord said:
Thou hast been mourning for them who should not be mourned for. Yet thou speakest words of wisdom. The (truly) wise grieve neither for the living nor the dead.

It is not that I have never existed, nor thou, nor these kings. Nor is it that we shall cease to exist in the future.

As are childhood, youth, and old age, in this body, to the embodied soul, so also is the attaining of another body. Calm souls are not deluded thereat.

Notions of heat and cold, of pain and pleasure, are born, O son of Kunti, only of the contact of the senses with their objects. They have a beginning and an end. They are impermanent in their nature. Bear them patiently, O descendant of Bharata.

That calm man who is the same in pain and pleasure, whom these cannot disturb, alone is able, O great amongst men, to attain to immortality.

The unreal never is. The Real never is not. Men possessed of the knowledge of the Truth fully know both these.

That by which all this is pervaded,—That know for certain to be indestructible. None has the power to destroy this Immutable.

Of this indwelling Self, the ever-changeless, the indestructible, the illimitable,—these bodies are said to have an end. Fight therefore, O descendant of Bharata.

He who takes the Self to be the slayer, he who takes It to be the slain, neither of these knows. It does not slay, nor is It slain.

This in never born, nor does It die. It is not that not having been It again comes into being. (Or according to another view: It is not that having been It again ceases to be). This is unborn, eternal, changeless, ever-Itself. It is not killed when the body is killed.