Bhagavad Gita 6.35
Lord Shri Krishna replied: Doubtless, O Mighty One, the mind is fickle and exceedingly difficult to restrain; however, O Son of Kunti, with practice and renunciation, it can be done.
श्री भगवानुवाच
असंशयं महाबाहो मनो दुर्निग्रहं चलं।
अभ्यासेन तु कौन्तेय वैराग्येण च गृह्यते॥śrī bhagavānuvāca
asaṃśayaṃ mahābāho mano durnigrahaṃ calaṃ|
abhyāsena tu kaunteya vairāgyeṇa ca gṛhyate||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).