Paramhansa Yogananda in The Essence of Self-Realization, On Meditation, by Swami Kriyananda

 Once, after a meditation at his desert retreat, Yogananda said to the disciples who were present, “This is the kingdom of OM. Listen! It is not enough merely to hear OM. You must merge yourself in that inner sound.

“OM is Divine Mother. OM Kali! OM Kali! OM Kali! Listen: Oh, how beautiful it is! OM Kali! OM Kali! OM Kali!”