Śāṇḍilya Upaniṣad 1.7_2

1.7_2

yuktaṃ yuktaṃ tyajedvāyuṃ yuktaṃ yuktaṃ ca pūrayet |
yuktaṃ yuktaṃ ca badhnīyādevaṃ siddhimavāpnuyāt ||

"He should (as far as is consistent with his health and safety) properly exhale it, properly inhale it or retain it properly. Thus (only) will he attain success. By thus retaining the breath in an approved manner and by the purification of the nādis, the brightening of the (gastric) fire, the hearing distinctly of (spiritual) sounds and (good) health result. When the nervous centres have become purified through the regular practice of Prāṇāyāma, the air easily forces its way up through the mouth of the Sushumnā which is in the middle. By the contraction of the muscles of the neck and by the contraction of the one below (viz.,) Apāna, the Prāṇa (breath) goes into the Sushumnā which is in the middle from the west nādi. Drawing up the Apāna and forcing down the Prāṇa from the throat, the yogin free from old age becomes a youth of sixteen.