Śaṅkaraon Brahma Sūtra 4.1.10

Authoritative authors also teach in their Sm;^/tis that a sitting posture subserves the act of meditation : cp. e.g. Bha. Gitd VI, ii, ‘Having made a firm seat for one’s self on a pure spot.’ For the same reason the Yogajastra teaches different sitting postures, viz. the so-called lotus position and so on.