Mani mantra

The six syllable mantra of Avalokiteshvara is oṃ mani padme hūṃ (pronounced by om mani pémé hung by Tibetans). In the terma tradition, Avalokiteshvara's seed syllable hrīḥ is added to the end of the mantra, making seven syllables.

Benefits of the Mani mantra

It is said:

"Reciting oṃ mani padme hūṃ one hundred or one thousand times is of equal merit to reciting the whole of the Kangyur.
Reciting oṃ mani padme hūṃ ten thousand times closes the door to rebirth in the lower realms.
Reciting oṃ mani padme hūṃ one million times one will reach the level of an irreversible bodhisattva.
Reciting oṃ mani padme hūṃ ten million times one will attain perfect buddhahood."

--See also The Tibetan Book of Living and Dying, Appendix Four, pp. 396-8