Bhagavadgita 2.21-30

He that knows This to be indestructible, changeless, without birth, and immutable, how is he, O son of Prithâ, to slay or cause another to slay?

Even as a man casts off worn-out clothes, and puts on others which are new, so the embodied casts off worn-out bodies, and enters into others which are new.

This (Self), weapons cut not; This, fire burns not; This, water wets not; and This, wind dries not.

This Self cannot be cut, nor burnt, nor wetted, nor dried. Changeless, all-pervading, unmoving, immovable, the Self is eternal.

This (Self) is said to be unmanifested, unthinkable, and unchangeable. Therefore, knowing This to be such, thou oughtest not to mourn.

But if thou shouldst take This to have constant birth and death, even in that case, O mighty-armed, thou oughtest not to mourn for This.

Of that which is born, death is certain, of that which is dead, birth is certain. Over the unavoidable, therefore, thou oughtest not to grieve.

All beings are unmanifested in their beginning, O Bhârata, manifested in their middle state and unmanifested again in their end. What is there then to grieve about?

Some look upon the Self as marvellous. Others speak of It as wonderful. Others again hear of It as a wonder. And still others, though hearing, do not understand It at all.

This, the Indweller in the bodies of all, is ever indestructible, O descendant of Bharata. Wherefore thou oughtest not to mourn for any creature.