Chapter 6 · verse 26 · gita.yoga.org

Bhagavad Gita 6.26

यतो यतो निश्चरति मनश्चञ्चलमस्थिरम्। ततस्ततो नियम्यैतदात्मन्येव वशं नयेत्

From whatever cause the restless and unsteady mind wanders away, let him restrain it from that and bring it under the control of the Self alone.