Bhagavad Gita, Chapter 6, Verse 13

Let him firmly hold his body, head and neck erect and still, with the eye-balls fixed, as if gazing at the tip of his nose, and not looking around.

Where the eyes settle turns out to matter more than it might seem, a wandering gaze tends to bring a wandering mind along with it.

This gets specific about where attention physically rests during practice. A fixed, steady gaze is offered here as a concrete anchor, something the mind can hold onto instead of drifting after whatever catches the eye.