Bhagavad Gita · 4.29

अपाने जुह्वति प्राणं प्राणेऽपानं तथापरे।प्राणापानगती रुद्ध्वा प्राणायामपरायणाः॥

apāne juhvati prāṇaṁ prāṇe 'pānaṁ tathāpare prāṇāpāna-gatī ruddhvā prāṇāyāma-parāyaṇāḥ

Others offer the outgoing breath into the incoming breath, and the incoming breath into the outgoing breath, restraining the movement of both breaths, fully devoted to the practice of breath control.

— Bhagavad Gita 4.29

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)