Bhagavad Gita, Chapter 6, Verse 12

There, seated on that seat, making the mind one-pointed and subduing the action of the imaging faculty and the senses, let him practise Yoga for the purification of the heart.

A steady, upright posture isn't just formality, it's part of what actually keeps the mind from drifting.

This connects body and mind directly. Sitting firmly, without slouching or fidgeting, is described as genuinely supporting the inner focus the rest of this chapter is building toward, not a separate concern from it.