Bhagavad Gita 6.34

My Lord! Verily, the mind is fickle and turbulent, obstinate, and strong; indeed, it is extremely difficult to control, like the wind.

चञ्चलं हि मनः कृष्ण प्रमाथि बलवद्दृढम्।
तस्याहं निग्रहं मन्ये वायोरिव सुदुष्करम्॥

cañcalaṃ hi manaḥ kṛṣṇa pramāthi balavaddṛḍham|
tasyāhaṃ nigrahaṃ manye vāyoriva suduṣkaram||

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