Bhagavad Gita 6.34

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

cañcalaṃ hi mana: kṛṣṇa pramāthi balavad-dṛḍham
tasyāhaṃ nigrahaṃ manye vāyor-iva suduṣkaram – 34 

The mind, O Krishna, is given to swinging and swerving. It is turbulent, very powerful too. To bring it under control or moderation is like restraining the wind.