Ancient Indian scriptures such as the Upanisads (Mundaka, Mandukya, Svetasvatara & Katha); Patanjali’s Yoga Sutra; and the Bhagvad Gita all have mentioned it in great detail.
Yoga Sutra 1.27, “Tasya Vachakah Pranavah”, meaning ॐ or AUM (Pranava) is the word denoting supreme consciousness or God. Besides being easier to pronounce, it is very practical to use. Spirituality lays emphasis on Aum because ancient yogis discovered that it is a useful sacred syllable that alters the state of consciousness and helps spiritual progress towards realizing the ultimate union with the highest reality.

Om or Aum is called the bija sound (seed), ‘the sound’ from which all other sounds of the world are generated. Aum is considered to represent God, Creator, and Universal Consciousness because it has the power to create everything.
Various scientific studies done on AUM suggest that the repetition of Aum results in improved physiological functioning. It has a calming effect on the psychological level. Also known as the ‘sound of the universe,’ since various countries recorded the same sound waves and frequency (by way of using various scientific measuring instruments sent out of earth’s orbit.)
A simple breathing exercise can be done anytime and anywhere. Aum can be done in two ways chanted out loud or can be done internally.
Chanting out loud- Sit in any meditative posture like Sukhasana or padmasana. Take a deep breath hold and exhale as long as you can. Chanting should be as loud as possible. Starts with A to U and ends with Mmmm (making a sound like a bumble bee.) Ensure stomach is empty. This is known as rechaka pranayama.
Internal Chanting- As you inhale internally chant A, U and while you are exhaling Mmm should be chanted in the mind.