The Upanishads

From https://www.ancient.eu/Upanishads/
The Upanishads are a collection of texts of religious and philosophical nature, written in India probably between c. 800 BCE and c. 500 BCE, during a time when Indian society started to question the traditional Vedic religious order.

https://en.wikipedia.org/wiki/Historical_Vedic_religion
The historical Vedic religion (also known as Vedism, Brahmanism, Vedic Brahmanism, and ancient Hinduism[note 1]) refers to the religious ideas and practices among Indo-Aryan-speaking peoples of ancient India after about 1500 BCE.[2][3][4] These ideas and practices are found in the Vedic texts, and they were one of the major influences that shaped contemporary Hinduism.[2] According to the Encyclopædia Britannica, in the Hindu tradition and particularly in India, the Vedic religion is considered to be a part of Hinduism