Bhagavad Gītā Chapter Five verse 27

sparśān kṛtvā bahir bāhyāṃś cakṣuścaivāntare bhruvoḥ |
prāṇāpānāu samāu kṛtvā nāsābhyantaracāriṇāu ||

samā - equal, constant; fair, impartial towards; like to or identical withprāṇa - vital force; the breath of life; breath; inward airapāna - downward air

Commentaries and Reflections

Commentary by T Krishnamacharya:

“Use Samavṛtti in Prāṇāyāma to reduce the power of Rajas and Tamas.”