Upanishad

“To sit down close to (one’s teacher).” This is a reference to the mode in which esoteric knowledge is transmitted by word of mouth from teacher (guru) to disciple (shishya). Traditionally, 108 such works are spoken of. They are the esoteric continuation of the Vedic ritualism. The earliest of them were composed in the era prior to Buddhism, dating as far back perhaps as the middle of the second millennium B.C.