In the Chandogya Upanishad it is said:
The essence of all beings is the earth.
The essence of the earth is water.
The essence of water is the plant.
The essence of the plant is man.
The essence of man is speech.
The essence of speech is the Rigveda.
The essence of Rigveda is the Samveda.
The essence of Samveda is OM.
Thus OM is the best of all essences, deserving the highest place. Visually, OM is represented by a stylized pictograph.
via Om – An Inquiry into its Aesthetics, Mysticism, and Philosophy.