The Bhagavad Gita is part of the Mahabharata which was compiled between 500 BCE and 500 CE. It has 18 chapters.
Krisna said to Arjuna:
While working towards yoga, remain detached from, and renounce all sense of purpose.View friends, comrades, enemies and neutrals all in the same way. Work to integrate with the Self: practice with a steady seat in a clean place, make the mind one-pointed, restrain thought and senses and engage in yoga to purify the Self. Be moderate in food, deeds, sleep and control thought, still the mind, to reach integration of the Self. Upon such a man of yoga the highest joy descends and Brahman he becomes.
KRISHNA
Reverence for the gods of light, for the twice-born, for the teachers of the spirit and for the wise; and also purity, righteousness, chastity and non-violence: this is the harmony of the body.
Words which give peace, words which are good and beautiful and true, and also the reading of sacred books: this is the harmony of words.
Quietness of mind, silence, self-harmony, loving-kindness, and a pure heart: this is the harmony of the mind.
This three-fold harmony is called pure when it is practiced with supreme faith with no desire for a reward and with oneness of soul.