Yōga Sūtra 2.49

तस्मिन्सति श्वासप्रश्वासयोर्गतिविच्छेदः प्राणायामः।

tasminsati śvāsapraśvāsayōrgativicchēdaḥ prāṇāyāmaḥ।

[तस्मिन् सति श्वास प्रश्वासयोः गति विच्छेदः प्राणायामः।
tasmin = with it; sati = along accomplish; śvāsa = inhalation; praśvāsayōḥ = exhalation; gati = pace of; vicchēdaḥ = breaking; prāṇāyāmaḥ = is prānāyāma, the regulation of breath;]

Along with (steady āsana) accomplish breaking the pace of inhalation and exhalation - this is prānāyāma, the regulation of breath.