Sacred syllable om
Bhagavad-gita As It Is
BG Chapters 7 - 12
BG 8.13, Translation and Purport:
After being situated in this yoga practice and vibrating the sacred syllable oṁ, the supreme combination of letters, if one thinks of the Supreme Personality of Godhead and quits his body, he will certainly reach the spiritual planets.
It is clearly stated here that oṁ, Brahman and Lord Kṛṣṇa are not different. The impersonal sound of Kṛṣṇa is oṁ, but the sound Hare Kṛṣṇa contains oṁ. The chanting of the Hare Kṛṣṇa mantra is clearly recommended for this age.