Yoga Sutras of Patanjali · 1.5
वृत्तयः पञ्चतय्यः क्लिष्टाक्लिष्टाः ॥५॥
vṛttayaḥ pañcatayyaḥ kliṣṭākliṣṭāḥ
The modifications are of five kinds, and each may be painful or non-painful.
— Yoga Sutras of Patanjali 1.5
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).