Answer:
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 .
