The Vedas are the fountainhead of all Indian philosophical tradition and spiritual practice. The teachings in the Rig Veda were preserved over centuries by oral tradition alone and was put into writing ~1700–1100 BCE - one of the world's oldest spiritual, philosophical and scientific texts. The teachings are broken down into detailed external practices, different forms of spiritual worship, and finally an explanation of what we realize when we do the practices.
Diving into ancient texts can be intense, so I recommend beginning with the Upanishads, what we can experience through dedication the practice of yoga (meditation), as known by the ancients.