Bhagavad Gita 6.36
It is not possible to attain Self-Realization unless one knows how to control oneself; however, for one who, striving by the proper means, learns such control, it is possible.
असंयतात्मना योगो दुष्प्राप इति मे मतिः।
वश्यात्मना तु यतता शक्योऽवाप्तुमुपायतः॥asaṃyatātmanā yogo duṣprāpa iti me matiḥ|
vaśyātmanā tu yatatā śakyo'vāptumupāyataḥ||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).