Bhagavad Gita 6.34

चञ्चलं हि मन: कृष्ण प्रमाथि बलवद्दृधम् |
तस्याहं निग्रहं मन्ये वायोरिव सुदुष्करम्

Translation

The mind is very restless, turbulent, strong and obstinate, O Krishna. It appears to me that it is more difficult to control than the wind.