Bhagwad Gita
Chapter 6, Verse 34
चञ्चलं हि मनः कृष्ण प्रमाथि बलवद्दृढम्।
तस्याहं निग्रहं मन्ये वायोरिव सुदुष्करम्।।6.34।।
For the mind is restless, turbulent, obstinate and very strong, O Krsna, and to subdue it is, it seems to me, more difficult than controlling the wind.