Madhvaon Brahma Sūtra 4.1.8

The Narayana Tantra has the following : “ Contem- plation is of two kinds, one of remembrance, the other of deep meditation. The former (the constant remembering of the qualities of Brahman) is fit to be practised always, the latter (deep meditation) is possible (only) in the sit. ting posture. Meditation is explained by the wise to be uninterrupted course of conscious states, and this is possible only for him who sits up (in complete wakeful- ness), but not for him who is lying in bed overcome by sleep, or standing or walking; for then distraction would necessarily set iti. Meditation is far superior to (mere) remembrance ; and this is beyond a doubt. Hence the necessity for the sitting posture, especially the practice being meditation.” Hence, it being meditation, (the necessity for the sitting posture, etc, is proved).