Bhagwad Gita
Chapter 6, Verse 25
शनैः शनैरुपरमेद् बुद्ध्या धृतिगृहीतया।
आत्मसंस्थं मनः कृत्वा न किञ्चिदपि चिन्तयेत्।।6.25।।
Gradually, step by step, with full conviction, one should become situated in trance by means of intelligence, and thus the mind should be fixed on the Self alone and should think of nothing else.