Bhagavad Gita
Chapter 6 · Verse 19
यथा दीपो निवातस्थो नेङ्गते सोपमा स्मृता। योगिनो यतचित्तस्य युञ्जतो योगमात्मनः॥
yathā dīpo nivāta-stho neṅgate sopamā smṛtā, yogino yata-cittasya yuñjato yogam ātmanaḥ
A well-known image for a trained mind. Not motionless because nothing happens around it, but steady because it's learned not to flicker at every draft.
A steady mind is a lamp that doesn't flicker, even in the wind.