Bhagwad Gita
Chapter 6, Verse 26
यतो यतो निश्चरति मनश्चञ्चलमस्थिरम्।
ततस्ततो नियम्यैतदात्मन्येव वशं नयेत्।।6.26।।
From whatever and wherever the mind wanders due to its flickering and unsteady nature, one must certainly withdraw it and bring it back under the control of the Self.