Amṛtanāda (Amṛtanādabindu) Upaniṣad 20

20

nāsikāpuṭamaṅgulyā pidhāyaikena mārutam |
ākṛṣya dhārayedagniṃ śabdamevābhicintayet ||

Seating himself on the ground on a seat of kuśa grass which is pleasant and devoid of all evils, having protected himself mentally (from all evil influences), uttering raṭha-maṇdala, assuming either paḍma, svasṭika, or bhaḍra posture or any other which can be practised easily, facing the north and closing the nostril with the thumb, one should inspire through the other nostril and retain breath inside and preserve the Agni (fire). Then he should think of the sound (Om) alone. Om, the one letter is Brahman; Om should not be breathed out. Through this divine manṭra (Om), it should be done many times to rid himself of impurity. Then as said before, the manṭra-.knowing wise should regularly meditate, beginning with the navel upwards in the gross, the primary (or less) gross and subtle (states). The greatly wise should give up all (sight) seeing across, up or down, and should practise yoga always being motionless and without tremor. The union as stated (done) by remaining without tremor in the hollow stalk (viz., Sushumnā) alone is ḍhāraṇā. The yoga with the ordained duration of twelve māṭrās is called (ḍhāraṇā). That which never decays is Akshara (Om) which is without ghosha (third, fourth, and fifth letters from K), consonant, vowel, palatal, guttural, nasal, letter R and sibilants. Prāṇa travels through (or goes by) that path through which this Akshara (Om) goes. Therefore it should be practised daily, in order to pass along that (course). It is through the opening (or hole) of the heart, through the opening of vāyu (probably navel), through the opening of the head and through the opening of moksha. They call it bila (cave), sushira (hole), or maṇdala (wheel).