Bhagavad Gita · 6.35

श्रीभगवानुवाचअसंशयं महाबाहो मनो दुर्निग्रहं चलम्।अभ्यासेन तु कौन्तेय वैराग्येण च गृह्यते॥

śhrī bhagavān uvācha asaṁśhayaṁ mahā-bāho mano durnigrahaṁ chalam abhyāsena tu kaunteya vairāgyeṇa cha gṛhyate

The Blessed Lord said: Undoubtedly, O mighty-armed one, the mind is restless and difficult to control. But it can be restrained through practice (abhyasa) and detachment (vairagya), O son of Kunti.

— Bhagavad Gita 6.35

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)