Krishna,Geeta,Chapter 6, Verse 11-12.
(for Yogis)

To practice yoga, one should go to a secluded place and should lay kusa-grass on the ground and then cover it with a deerskin and a soft cloth. The seat should neither be too high nor too low and should be situated in a sacred place. The yogi should then sit on it very firmly and should practice yoga by controlling the mind and the senses, purifying the heart and fixing the mind on one point.