The key concept of death and rebirth cycles (reincarnation) in Hinduism is expressed in this sacred text: Arayankes Bhagavad Gita Upanishads Sanskrit

The key concept of death and rebirth cycles (reincarnation) in Hinduism is expressed in this sacred text : Upanishads.

Explanation:

Upanishads -

The concept of samsara which explains the cycle of life has roots in Vedic literature, but this theory is not discussed in it. It appears as a developed form, but without any mechanistic details, in early Upanishads

And, Although the concept of rebirth has been mentioned in hind literature as old as Vedas (Rigveda), it has been dealt in much detail in Upanishads.