Nadabindu Upanishad Verse 56

Sanskrit

दृष्टिः स्थिरा यस्य विना सदृश्यं वायुः स्थिरो यस्य विना प्रयत्नम् । चित्तं स्थिरं यस्य विनावलम्बं स ब्रह्मतारान्तरनादरूपः ॥ ५६॥

K. Narayanasvami Aiyar's Translation

When the (spiritual) sight becomes fixed without any object to be seen, when the vāyu (prāṇa) becomes still without any effort, and when the citta becomes firm without any support, he becomes of the form of the internal sound of Brahma-Praṇava.

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Reference: Nadabindu Upanishad