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.

haṭha - force, forcefulaṅga - limbāsana - posturepūrva - precedingtat - thatsthairya - steadfastness, steadinessārogya - absence of illnessca - andlāghava - lightness

Commentaries and Reflections

Paul Harvey:

 

http://www.yogastudies.org/2015/05/asana-brings-steadiness-health-and-lightness/