Bhagavad Gita 6.26

When the volatile and wavering mind wanders, let him restrain it and bring it back to its allegiance to the Self.

यतो यतो निश्चरति मनश्चञ्चलमस्थिरम्।
ततस्ततो नियम्यैतदात्मन्येव वशं नयेत्॥

yato yato niścarati manaścañcalamasthiram|
tatastato niyamyaitadātmanyeva vaśaṃ nayet||

English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).