Nadabindu Upanishad Verse 41

Sanskrit

सर्वचिन्तां समुत्सृज्य सर्वचेष्टाविवर्जितः । नादमेवानुसंदध्यान्नादे चित्तं विलीयते ॥ ४१॥

K. Narayanasvami Aiyar's Translation

Having abandoned all thoughts and being freed from all actions, he should always concentrate his attention on the sound, and (then) his citta becomes absorbed in it.

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Reference: Nadabindu Upanishad