Bhagavad Gita · 6.25

शनैः शनैरुपरमेद्बुद्ध्या धृतिगृहीतया।आत्मसंस्थं मनः कृत्वा न किञ्चिदपि चिन्तयेत्॥

śhanaiḥ śhanair uparamed buddhyā dhṛti-gṛhītayā ātma-saṁsthaṁ manaḥ kṛtvā na kiñchid api chintayet

Gradually, step by step, one should attain tranquility with the intellect held firm by conviction. Having fixed the mind on the Self, one should think of nothing else whatsoever.

— Bhagavad Gita 6.25

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