Bhagavad Gita 4.29
There are some who practice controlling the vital energy and governing the subtle forces of prana and apana, thereby sacrificing their prana to apana, or their apana to prana.
अपाने जुह्वति प्राण प्राणेऽपानं तथाऽपरे।
प्राणापानगती रुद्ध्वा प्राणायामपरायणाः॥apāne juhvati prāṇa prāṇe'pānaṃ tathā'pare|
prāṇāpānagatī ruddhvā prāṇāyāmaparāyaṇāḥ||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).