Bhagavad Gita 6.25

शनै: शनैरूपमेद्बुद्ध्या धृतिगृहितया |
आत्मसंस्थं मनः कृत्वा न किञ्चिदपि चिन्तयेत्

Translation

Slowly and steadily, with conviction in the intellect, the mind will become fixed in God alone, and will think of nothing else.