Yoga Sutras of Patanjali · 2.49

तस्मिन्सति श्वासप्रश्वासयोर्गतिविच्छेदः प्राणायामः ॥४९॥

tasmin sati śvāsa-praśvāsayor gati-vicchedaḥ prāṇāyāmaḥ

With that established, the regulation of inhalation and exhalation is pranayama.

— Yoga Sutras of Patanjali 2.49

Source: Sanskrit attributed to Patanjali (c. 2nd century BCE – 4th century CE). Public-domain English readings drawn from Swami Vivekananda's 'Raja Yoga' (1896) and Charles Johnston's 'The Yoga Sutras of Patanjali' (1912).