Katha-Upanishad
Om is the supreme mantra (mystical sound) of Hindu origin, but sacred and important to various religions of Dharmic origin (Hinduism, Buddhism, Jainism…). It is the sound vibration of the Absolute, which lies beneath everything manifested (physical plane, causal plane, subtle plane). It is the sound of silence, the sound of the Self, the sound of Being. The sound “A” represents Creation, like Brahma. The sound “U” represents Conservation, like Vishnu. The sound “M” represents Dissolution, like Shiva. Om is the all encompassing vibration of the Universe.
Mandookya-Upanishad
Om. Om. Om.