Bg 6.19

yathā dīpo nivāta-stho
neṅgate sopamā smṛtā
yogino yata-cittasya
yuñjato yogam ātmanaḥ
Synonyms: 
yathā — as; dīpaḥ — a lamp; nivāta-sthaḥ — in a place without wind; na — does not; iṅgate — waver; — this; upamā — comparison; smṛtā — is considered; yoginaḥ — of the yogī; yata-cittasya — whose mind is controlled; yuñjataḥ — constantly engaged; yogam — in meditation; ātmanaḥ — on transcendence.
Translation: 
As a lamp in a windless place does not waver, so the transcendentalist, whose mind is controlled, remains always steady in his meditation on the transcendent Self.
Purport: 

A truly Kṛṣṇa conscious person, always absorbed in transcendence, in constant undisturbed meditation on his worshipable Lord, is as steady as a lamp in a windless place.