Bhagavad Gita 6.25
Little by little, with the help of his reason controlled by fortitude, let him attain peace; and, fixing his mind on the Self, let him not think of anything else.
शनैः शनैरुपरमेद् बुद्ध्या धृतिगृहीतया।
आत्मसंस्थं मनः कृत्वा न किञ्चिदपि चिन्तयेत्॥śanaiḥ śanairuparamed buddhyā dhṛtigṛhītayā|
ātmasaṃsthaṃ manaḥ kṛtvā na kiñcidapi cintayet||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).