Bhagavad Gita 6.13

समं कायशिरोग्रीवं धारयन्नचलं स्थिर: |
सम्प्रेक्ष्य नासिकाग्रं स्वं दिशश्चानवलोक्यन्

Translation

He must hold the body, neck, and head firmly in a straight line, and gaze at the tip of the nose, without allowing the eyes to wander.