Bhagavad Gita · 6.19

यथा दीपो निवातस्थो नेङ्गते सोपमा स्मृता।योगिनो यतचित्तस्य युञ्जतो योगमात्मनः॥

yathā dīpo nivāta-stho neṅgate sopamā smṛtā yogino yata-chittasya yuñjato yogam ātmanaḥ

Just as a lamp in a windless place does not flicker — that is the simile used for the yogi of controlled mind who practices meditation on the Self.

— Bhagavad Gita 6.19

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)