Chapter 4 · Knowledge and Renunciation of Action · Krishna said
Bhagavad Gita 4.29 — Breath as offering
अपाने जुह्वति प्राणं प्राणेऽपानं तथापरे। प्राणापानगती रुद्ध्वा प्राणायामपरायणाः ॥
apāne juhvati prāṇaṃ prāṇe'pānaṃ tathāpare | prāṇāpānagatī ruddhvā prāṇāyāmaparāyaṇāḥ ||
Meaning
Others, devoted to breath control, offer the outgoing breath into the incoming and the incoming into the outgoing, holding the flow of both.