The mantra that is held very sacred by all Hindus is the mantra Om also called praNava mantra. This mantra is part of every worship. This mantra is hailed in scriptures of various languages. vedas - the holy scripture - is chanted always after chanting this mantra and always concluded with this mantra. Each of the hailing in thearchanai that is performed for the divine is preceded by this mantra Om.

praNava is the primal sound that existed before the creation and the sound that stays after the praLaya. This is the nAtharUpa - the Form of sound, of the Supreme Luminance. This mantra om refer to none other than God. The great yogis meditate in this mantra as the path to Eternal Bliss of the Formless Nameless God !

The praNava has five parts. They are akara, ukara, makara, bindhu, nAtham. (aum). Consequently this is also one of the glorious panchAkshara mantra (1).

The scriptures that explain the glory of this praNava mantra are quite many. However the significant one is from the skandha purANa. It is a well known purANic event that skandha described the real meaning of the praNava mantra to God shiva. In the above mentioned episode of skandha purANa the sage vAmadeva seeks the boon from Lord skandha to tell him too that description. Lord skandha who was pleased with the devotion of sage vAmadeva told him that great secret.(2)

Udgita upasana - Significance of Om from Chandogya Upanishad

See Also: 
1. panchAkshara mantra 
2. Explanation of praNava mantra from skandha purANa