Haṭha Yoga Pradīpikā Chapter One verse 17

हठस्य प्रथमाङ्गत्वाद् आसनं पूर्वम् उच्यते ।
कुर्यात् तद् आसनं स्थैर्यम् आरोग्यं चाङ्गलाघवम् ॥१७॥

haṭhasya prathama aṅgatvād āsanaṁ pūrvam ucyate |
kuryāt tad āsanaṃ sthairyam ārogyaṃ cāṅga lāghvam ||

Āsana brings steadiness, health and lightness of limb.

haṭha - force, forcefulāsana - particular posture; sitting, sitting down; seat, place, stool; abiding, dwellingpūrva - precedingtat - that, thissthairya - steadfastness, steadinessārogya - absence of illnessca - andaṅga - limblāghava - lightness

Commentaries and Reflections

Paul Harvey:

“For me, still to this day, one of the finest, simplest, direct and most succinct definitions on the purpose of Āsana within the processes and practices of Haṭha Yoga, is the definition offered in the Haṭha Yoga Pradīpikā Chapter 1 verse 17. It is a definition valid for any situation, or presentation, or as a response to questions from any background or level of interest around why we practice Āsana.
It can also be a springboard to discussing physiological qualities such as the relationship of Agni to the energetic qualities of health and lightness of limb. Or it can be a springboard to discussing psychological qualities such as the relationship of the GuṇaRajas, to mental qualities such as steadiness.”