Holding the body steady with the three (upper parts – chest, neck and head) erect, causing the senses and the mind to enter into the heart, the wise man should cross by the boat of Brahman all the streams which cause fear.
Repressing his breathings here (in the body), let him who has controlled all movements, breathe through his nostrils, with diminished breath; let the wise man restrain his mind vigilantly as (he would) a chariot yoked with vicious horses.
In a level clean place, free from pebbles, fire and gravel, favorable to thought by the sound of water and other features, not offensive to the eye, in a hidden retreat protected from the wind, let him practice yoga.
Fog, smoke, sun, wind, fire, fireflies, lightning, crystal moon, these are the preliminary forms which produce the manifestation of Brahman in Yoga.
When the fivefold quality of Yoga is produced, as earth, water, fire, air and ether arise, then there is no longer sickness, no old age, no death to him who has obtained a body made of the fire of Yoga.
Lightness, healthiness, steadiness, clearness of complexion, pleasantness of voice, sweetness of odor, and slight excretions, these, they say are the first results of the progress of yoga.
Svetasvatara Upanishad, Chapter II – Verses (8 – 13)
(Source: The Ten Principal Upanishads by S. Radhakrishnan)