Yoga Sutras of Patanjali · 2.50

बाह्याभ्यन्तरस्तम्भवृत्तिर्देशकालसंख्याभिः परिदृष्टो दीर्घसूक्ष्मः ॥५०॥

bāhyābhyantara-stambha-vṛttir deśa-kāla-saṁkhyābhiḥ paridṛṣṭo dīrgha-sūkṣmaḥ

Pranayama has three movements — outward, inward, and held in suspension — regulated by place, time, and number, and becoming long and subtle.

— Yoga Sutras of Patanjali 2.50

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).