In the Bhagavad Gita (6.19), Lord Krishna says:

"Yathā dīpo nivātastho neṅgate sopamā smṛtā,
Yogino yata-cittasya yuñjato yogam ātmanaḥ."

Meaning:
Just as a lamp placed in a windless place does not flicker,
so is the mind of a yogi, steady in meditation upon the Self.