Bhagavad Gita · Chapter 6: The Path of Meditation

Bhagavad Gita 6.26

yato yato niścarati manaścañcalamasthiram | tatastato niyamyaitadātmanyeva vaśaṃ nayet ||

Wherever the restless, unsteady mind wanders off, from there one should pull it back and bring it under the control of the self alone.

Modern context

The mind will wander. Krishna simply assumes it, and that is a comfort. The instruction is not to be shocked when it happens, but to notice where it went and bring it back, and to do that again and again. This returning is the practice. Each time you notice the wandering and come back, the muscle of attention gets a little stronger. A session full of wandering and returning is not a failed session.