OM

The most sacred syllable in Hinduism is OM.It stands for Brahman both as a personal and impersonal GOD.

The goal which all the Vedas declare, which all austerities aim at, and which men desire when they lead the life of continence, I will tell you briefly: it is OM. This syllable OM is indeed Brahman. This syllable is the Highest. Whosoever knows this syllable obtains all that he desires. This is the best support; this is the highest support. Whosoever knows this support is adored in the world of Brahma.

-- Katha Upanishads I, ii, 15-17 ¹
The sound OM is Brahman. The maharishis and sages practiced austerity to realize that Sound-Brahman. After attaining perfection one hears the sound of this eternal Word rising spontaneously from the navel. "'What will you gain', some sages ask, 'by merely hearing this sound?' You hear the roar of the ocean from a distance. By following the roar you can reach the ocean. As long as there is the roar, there must also be the ocean. By following the trail of OM you attain Brahman, of which the Word is the symbol. That Brahman has been described by the Vedas as the ultimate goal."

-- The Gospel of Sri Ramakrishna ²