तस्मिन्सति श्वासप्रश्वासयोर्गतिविच्छेदः प्राणायामः ॥ २.४९ ॥

(49) When that (yoga poses) is perfected, then comes the regulation of the flow of inhalation and exhalation - pranayama.

Patanjali's Yoga Sutra, Book II, Verse 49