The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
9. And with reference to immobility (scripture ascribes thought to the earth, &c.).
Moreover, in the passage 'The earth thinks as it were' scripture ascribes thought to the earth, with regard to its immobility. This also helps us to infer that meditation is the occupation of one who is sitting.