Mahabharata Bhishma Parva Chapter 26:2

Mahabharata Bhishma Parva (Bhagavat-Gita Parva) Chapter 26:2
Bhagavad Gita Chapter II

Know that [the soul] to be immortal by which all this [universe] is pervaded. No one can compass the destruction of that which is imperishable. It hath been said that those bodies of the Embodied (soul) which is eternal, indestructible and infinite, have an end. Do thou, therefore, fight, O Bharata. He who thinks it (the soul) to be the slayer and he who thinks it to be the slain, both of them know nothing; for it neither slays nor is slain. It is never born, nor doth it ever die; nor, having existed, will it exist no more. Unborn, unchangeable, eternal, and ancient, it is not slain upon the body being perished. That man who knoweth it to be indestructible, unchangeable, without decay, how and whom can he slay or cause to be slain? As a man, casting off robes that are worn out, putteth on others that are new, so the Embodied (soul), casting off bodies that are worn out, entereth other bodies that are new. Weapons cleave it not, fire consumeth it not; the waters do not drench it, nor doth the wind waste it. It is incapable of being cut, burnt, drenched, or dried up. It is unchangeable, all-pervading, stable, firm, and eternal. It is said to be imperceivable, inconceivable and unchangeable. Therefore, knowing it to be such, it behoveth thee not to mourn (for it). Then again even if thou regardest it as constantly born and constantly dead, it behoveth thee not yet, O mighty-armed one, to mourn (for it) thus. For, of one that is born, death is certain; and of one that is dead, birth is certain. Therefore it behoveth thee not to mourn in a matter that is unavoidable. All beings (before birth) were unmanifest.

Only during an interval (between birth and death), O Bharata, are they manifest; and then again, when death comes, they become (once more) unmanifest. What grief then is there in this? One looks upon it as a marvel; another speaks of it as a marvel. Yet even after having heard of it, no one apprehends it truly. The Embodied (soul), O Bharata, is ever indestructible in everyone's body. Therefore, it behoveth thee not to grieve for all (those) creatures. Casting thy eyes on the (prescribed) duties of thy order, it behoveth thee not to waver, for there is nothing else that is better for a Kshatriya than a battle fought fairly. Arrived of itself and (like unto) an open gate of heaven, happy are those Kshatriyas, O Partha, that obtain such a fight. But if thou dost not fight such a just battle, thou shalt then incur sin by abandoning the duties of thy order and thy fame. People will then proclaim thy eternal infamy, and to one that is held in respect, infamy is greater (as an evil) than death itself. All great car-warriors will regard thee as abstaining from battle from fear, and thou wilt be thought lightly by those that had (hitherto) esteemed thee highly.