The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
10. And Smriti-passages say the same.
Authoritative authors also teach in their Smritis that a sitting posture subserves the act of meditation: cp. e.g. Bha. Gîtâ VI. 11,--Having made a firm seat for one's self on a pure spot.' For the same reason the Yogasâstra teaches different sitting posture, viz. the so-called lotus position and so on.