Bhagavad Gita · 6.36

असंयतात्मना योगो दुष्प्राप इति मे मतिः।वश्यात्मना तु यतता शक्योऽवाप्तुमुपायतः॥

asaṁyatātmanā yogo duṣhprāpa iti me matiḥ vaśhyātmanā tu yatatā śhakyo 'vāptum upāyataḥ

Yoga is difficult to attain for one whose mind is uncontrolled, in My opinion. But it is achievable by one who has controlled the mind and strives through proper means.

— Bhagavad Gita 6.36

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